rawshooter

Since I found the YouTube comparison between the Sigma fp and the Blackmagic Pocket Cinema Camera 4K so worthless, I did a comparison test of my own.
I wanted to compare the cameras in two respects: 
(1) dynamic range when shooting a high-contrast subject under optimal light/with optimal exposure + robustness in grading the resulting image;
(2) low light/extreme high ISO image.
My setup was as follows:
Completely dark room; Both cameras with the same lens, Tokina 28-70mm/2.8 (Nikon mount adapted to L-mount and MFT respectively) at the same aperture setting (f4) on both cameras, but at 35mm focal length on the Pocket 4K and at 70mm on the Sigma fp to compensate for the different sensor sizes; Record settings: UHD 23.98p on both cameras, full frame CinemaDNG 12bit on the Sigma fp and BRAW Q0 on the Pocket 4K (= best quality codec settings on both cameras);  For the high-contrast test: daylight-temperature LED fresnel with maximum focus/spotlight on a plastic appliance reflecting some of the light to create high contrast; both cameras at minimum/native ISOs exposed ETTR/to the right. Both cameras set to 11.25 degrees shutter angle (in lieu of an ND filter, since motion rendering is irrelevant in this test).
(Note: Lowest, respectively native, ISOs on the two cameras are ISO 100 on the Sigma fp and ISO 400 on the Pocket 4K. Never mind that nominal difference, both cameras have almost identical clipping behavior at these settings, i.e. ISO 100 on the Sigma fp behaves like ISO 400 on the Pocket 4K. On both cameras, the zebras turned out to be reliable indicators for sensor clipping: To go absolutely sure that I would optimally expose the sensor, I also shot the scene at larger shutter angles - 22.5, 45, 90 and 172.5 degrees -, with zebras popping up as early as at 22.5 on both cameras. When looking at the material in Resolve, this was indeed where clipping had occurred and waveforms remained clipped in the RGB parade even when lowering exposure in Resolve's Raw control tab.)  For the low-light test: same 'scene' as above, but with the LED fresnel turned off and only a practical light in the background turned on. Both cameras set to maximum ISO (256.000), at f4 and 172.5 degrees shutter angle. Treatment in Resolve:
Basic image adjustments only in the Raw tab, interpretation in P3 color space with Rec. 709 gamma: adjustment of white balance/tint and exposure to make the Sigma fp and Pocket 4K footage match. (The raw material of the Blackmagic Pocket 4K was much warmer than that of the Sigma fp with the same Kelvin settings...) No highlight recovery. (Wasn't necessary anyway since there was no clipping in the images.) No noise filtering or sharpening, although the Pocket 4K's BRAW codec already has some baked-in temporal noise filtering. For the extreme grade, only a solarization-like custom curve was applied that pushed the shadows to the maximum and was meant to provoke banding in the material by pushing contrasts:
I exported 16bit TIFF screengrabs which can be downloaded here (6 TIFF files in a zip archive, 133 MB).
Here's how the well-exposed high-contrast scene looks like (25% downscaled images):

Sigma fp

Pocket 4K

- Note that the difference in sharpness may be my user error, and is also influenced by the different depth-of-field between 35mm/f4 on MFT and 70mm/f4 on full frame. The manual focus aides on the Pocket 4K are much better with latest firmware, so nailing focus without an external monitor was easier.

1:1 crops of the above two images:
 
Sigma fp (left) - Pocket 4K (right).
 
Extreme grade, with the same curve (as posted above) applied to the two above images:


Sigma fp

Pocket 4K
1:1 crops of the above:

Sigma fp

Pocket 4K
 
Low light, with both cameras at maximum (256,000) ISO, 172.5 degrees shutter and f4:

Sigma fp

Pocket 4K
1:1 crops of the above:

Sigma fp

Pocket 4K
So, to summarize, I think it's fair to say that the full-frame 12bit CinemaDNG material of the Sigma fp simply shows the benefit of a larger sensor and its lower image noise (even at base ISO if you compare the full-size TIFFs). It's thus only logical that it holds up better in extreme grades and in low light.
My likely user error in nailing the focus of Sigma fp also shows the strengths of the Pocket 4K, namely better camera assist functions and overally a better user interface/more practical user experience for video shooting. - But it's also nice to see that the Sigma fp has some genuine advantages over the Pocket 4K, especially for my own type of videomaking which revolves around event videos (concerts at indie/DIY venues) shot in extreme low light conditions.
 

Here's another dynamic range shootout between the Sigma fp and the Pocket 4K, this time with the LED fresnel at full power to create maximum contrast and a hard shadow, and an X-Rite Color Chart Pocket Video in the shadow to have a better indicator of shadow dynamic range. 
Cameras were set to 11.2 degrees shutter, ISO 100 (Sigma fp) respectively ISO 400 (Pocket 4K). With this harsh light, I needed to set the Pocket 4K to f5.6 for ETTR exposure (i.e. exposure just below the clipping point) while the Sigma fp clipped at f5.6 and needed to be stopped down to f8 for correct ETTR exposure. (This is an indicator for camera's native ISO being 800 in log. In other words, the Sigma fp is about one stop more light sensitive at ISO 100 than the Pocket 4K at ISO 400.)
This resulted in the following pictures, with DCI-P3 color space + Rec709 gamma interpretation in Resolve and Color Temp/Tint and exposure sliders adjusted to have the two camera images visually match:

Sigma fp

Pocket 4K
 
And extreme lift of the shadows with the following curve...

...reveals the following:
 

Sigma fp

Pocket 4K
1:1 crops of the upper left color patches:
 
Sigma fp

Pocket 4K

I'd say that on the Sigma fp, three of the dark grey color patches can still be differentiated whereas on the Pocket 4K, they are undifferentiated. Since I don't have a precise-enough meter to measure the difference of the reflected light, I'd say in a completely unscientific way that the fp has probably one stop more dynamic range in the shadows when the picture is exposed ETTR.
It also seems as if in the extreme shadows, the temporal noise filter of the Pocket 4K's BRAW really kicks in, since single pixels on the fp's CinemaDNG are clearly differentiated whereas on the Pocket 4K's image, they are blurred. (This is why in this scenario/grade, the Pocket 4K's image is [surprisingly] less noisy than the fp's.)  - EDIT: indeed, if you compare it to the DNG still from the Pocket 4K where BRAW's processing isn't applied:

 
- This time, I created a complete zip archive (91 MB) with one frame of the Sigma fp's CinemaDNG recording, the Pocket 4K's original BRAW file (consisting of just 3 frames) + a DNG shot with the Pocket 4K, plus the full-resolution UHD 16bit tiff of all above images: 
 
 

Okay, made another test with the setup you proposed:
Pocket 4K with 0.64x Metabones Speed Booster & Tokina 28-70mm/2.8 at 50mm and f2.8 [=actually at 32mm + f1.8], with camera exposed at ISO 128,000 versus the Sigma fp with the conventionally adapted, non-speedboosted Tokina 28-70mm/2.8 at 70mm and f2.8 and ISO 256,000 Images:

Pocket 4K BRAW (= with some in-camera noise reduction), ISO 128,000 + 0.64x Speed Booster

Pocket 4K DNG, ISO 128,000 + 0.64x Speed Booster

Sigma fp CinemaDNG, ISO 256,000
1:1 crops:

Pocket 4K BRAW  (= with some in-camera noise reduction), ISO 128,000 + 0.64x Speed Booster

Pocket 4K DNG, ISO 128,000 + 0.64x Speed Booster

Sigma CinemaDNG, ISO 256,000
 
The result is pretty much as to be expected, with the speedboosted Pocket 4K vs. the Sigma fp performing more like an APS-C camera vs. a full frame camera in low light with high ISO. As soon you expose on native ISO (without pushing gain), you however will back to MFT vs. full frame sensor performance in regards to noise levels.

Okay, made another test with the setup you proposed:
Pocket 4K with 0.64x Metabones Speed Booster & Tokina 28-70mm/2.8 at 50mm and f2.8 [=actually at 32mm + f1.8], with camera exposed at ISO 128,000 versus the Sigma fp with the conventionally adapted, non-speedboosted Tokina 28-70mm/2.8 at 70mm and f2.8 and ISO 256,000 Images:

Pocket 4K BRAW (= with some in-camera noise reduction), ISO 128,000 + 0.64x Speed Booster

Pocket 4K DNG, ISO 128,000 + 0.64x Speed Booster

Sigma fp CinemaDNG, ISO 256,000
1:1 crops:

Pocket 4K BRAW  (= with some in-camera noise reduction), ISO 128,000 + 0.64x Speed Booster

Pocket 4K DNG, ISO 128,000 + 0.64x Speed Booster

Sigma CinemaDNG, ISO 256,000
 
The result is pretty much as to be expected, with the speedboosted Pocket 4K vs. the Sigma fp performing more like an APS-C camera vs. a full frame camera in low light with high ISO. As soon you expose on native ISO (without pushing gain), you however will back to MFT vs. full frame sensor performance in regards to noise levels.

I made a series of exposures, by gradually opening the shutter and thus exposing beyond the clipping point indicated by the zebras.
So far, the zebras both on the Pocket 4K and the Sigma fp have been 100% reliable indicators of clipping. (At least with white light where RGB clipping is uniform.) Whenever zebras became visible on the camera display, I ended up having clipped waveforms in Resolve.
(Of course, "highlight reconstruction" needs to be switched off in Resolve in order to see sensor clipping.)
 

Here's another dynamic range shootout between the Sigma fp and the Pocket 4K, this time with the LED fresnel at full power to create maximum contrast and a hard shadow, and an X-Rite Color Chart Pocket Video in the shadow to have a better indicator of shadow dynamic range. 
Cameras were set to 11.2 degrees shutter, ISO 100 (Sigma fp) respectively ISO 400 (Pocket 4K). With this harsh light, I needed to set the Pocket 4K to f5.6 for ETTR exposure (i.e. exposure just below the clipping point) while the Sigma fp clipped at f5.6 and needed to be stopped down to f8 for correct ETTR exposure. (This is an indicator for camera's native ISO being 800 in log. In other words, the Sigma fp is about one stop more light sensitive at ISO 100 than the Pocket 4K at ISO 400.)
This resulted in the following pictures, with DCI-P3 color space + Rec709 gamma interpretation in Resolve and Color Temp/Tint and exposure sliders adjusted to have the two camera images visually match:

Sigma fp

Pocket 4K
 
And extreme lift of the shadows with the following curve...

...reveals the following:
 

Sigma fp

Pocket 4K
1:1 crops of the upper left color patches:
 
Sigma fp

Pocket 4K

I'd say that on the Sigma fp, three of the dark grey color patches can still be differentiated whereas on the Pocket 4K, they are undifferentiated. Since I don't have a precise-enough meter to measure the difference of the reflected light, I'd say in a completely unscientific way that the fp has probably one stop more dynamic range in the shadows when the picture is exposed ETTR.
It also seems as if in the extreme shadows, the temporal noise filter of the Pocket 4K's BRAW really kicks in, since single pixels on the fp's CinemaDNG are clearly differentiated whereas on the Pocket 4K's image, they are blurred. (This is why in this scenario/grade, the Pocket 4K's image is [surprisingly] less noisy than the fp's.)  - EDIT: indeed, if you compare it to the DNG still from the Pocket 4K where BRAW's processing isn't applied:

 
- This time, I created a complete zip archive (91 MB) with one frame of the Sigma fp's CinemaDNG recording, the Pocket 4K's original BRAW file (consisting of just 3 frames) + a DNG shot with the Pocket 4K, plus the full-resolution UHD 16bit tiff of all above images: 
 
 

Sure, here are 4 single DNG frames (35 MB zip archive) shot with both cameras in the two different light setups. The two DNGs of the Sigma fp were taken from the CinemaDNG folder, the two DNGs of the Blackmagic RAW were shot with the still function of the camera directly after shooting BRAW, with the same camera settings. (The BRAW files are too big to share here.)
To better judge tonality and dynamic range, I applied another 'extreme grade' solarisation curve which, this time, doesn't clip parts of the image and also compresses the dynamic range a bit so that highlight and shadow rendition become better visible:




Sigma fp

Pocket 4K
Full-resolution, 16bit TIFFs can be downloaded here (48 MB zip archive).
 
Highlight rendition is pretty identical with both cameras since they were both exposed to the right. So any difference in dynamic range should be visible in the shadows. A good indicator are the color gradations and red stripe on the lamp in the foreground that (still) can be seen in the low-light, low-contrast image:


In the high contrast light, neither the Sigma fp nor the Pocket 4K render these details anymore, but turn the lamp into one dark monchrome blob (with more image noise in the case of the Pocket 4K). So I think it's fair to say that both cameras have about the same dynamic range at optimal ISO settings, only that the Sigma fp renders smoother color gradations because of its lower sensor noise, as visible in the extreme grade above.

 

Since I found the YouTube comparison between the Sigma fp and the Blackmagic Pocket Cinema Camera 4K so worthless, I did a comparison test of my own.
I wanted to compare the cameras in two respects: 
(1) dynamic range when shooting a high-contrast subject under optimal light/with optimal exposure + robustness in grading the resulting image;
(2) low light/extreme high ISO image.
My setup was as follows:
Completely dark room; Both cameras with the same lens, Tokina 28-70mm/2.8 (Nikon mount adapted to L-mount and MFT respectively) at the same aperture setting (f4) on both cameras, but at 35mm focal length on the Pocket 4K and at 70mm on the Sigma fp to compensate for the different sensor sizes; Record settings: UHD 23.98p on both cameras, full frame CinemaDNG 12bit on the Sigma fp and BRAW Q0 on the Pocket 4K (= best quality codec settings on both cameras);  For the high-contrast test: daylight-temperature LED fresnel with maximum focus/spotlight on a plastic appliance reflecting some of the light to create high contrast; both cameras at minimum/native ISOs exposed ETTR/to the right. Both cameras set to 11.25 degrees shutter angle (in lieu of an ND filter, since motion rendering is irrelevant in this test).
(Note: Lowest, respectively native, ISOs on the two cameras are ISO 100 on the Sigma fp and ISO 400 on the Pocket 4K. Never mind that nominal difference, both cameras have almost identical clipping behavior at these settings, i.e. ISO 100 on the Sigma fp behaves like ISO 400 on the Pocket 4K. On both cameras, the zebras turned out to be reliable indicators for sensor clipping: To go absolutely sure that I would optimally expose the sensor, I also shot the scene at larger shutter angles - 22.5, 45, 90 and 172.5 degrees -, with zebras popping up as early as at 22.5 on both cameras. When looking at the material in Resolve, this was indeed where clipping had occurred and waveforms remained clipped in the RGB parade even when lowering exposure in Resolve's Raw control tab.)  For the low-light test: same 'scene' as above, but with the LED fresnel turned off and only a practical light in the background turned on. Both cameras set to maximum ISO (256.000), at f4 and 172.5 degrees shutter angle. Treatment in Resolve:
Basic image adjustments only in the Raw tab, interpretation in P3 color space with Rec. 709 gamma: adjustment of white balance/tint and exposure to make the Sigma fp and Pocket 4K footage match. (The raw material of the Blackmagic Pocket 4K was much warmer than that of the Sigma fp with the same Kelvin settings...) No highlight recovery. (Wasn't necessary anyway since there was no clipping in the images.) No noise filtering or sharpening, although the Pocket 4K's BRAW codec already has some baked-in temporal noise filtering. For the extreme grade, only a solarization-like custom curve was applied that pushed the shadows to the maximum and was meant to provoke banding in the material by pushing contrasts:
I exported 16bit TIFF screengrabs which can be downloaded here (6 TIFF files in a zip archive, 133 MB).
Here's how the well-exposed high-contrast scene looks like (25% downscaled images):

Sigma fp

Pocket 4K

- Note that the difference in sharpness may be my user error, and is also influenced by the different depth-of-field between 35mm/f4 on MFT and 70mm/f4 on full frame. The manual focus aides on the Pocket 4K are much better with latest firmware, so nailing focus without an external monitor was easier.

1:1 crops of the above two images:
 
Sigma fp (left) - Pocket 4K (right).
 
Extreme grade, with the same curve (as posted above) applied to the two above images:


Sigma fp

Pocket 4K
1:1 crops of the above:

Sigma fp

Pocket 4K
 
Low light, with both cameras at maximum (256,000) ISO, 172.5 degrees shutter and f4:

Sigma fp

Pocket 4K
1:1 crops of the above:

Sigma fp

Pocket 4K
So, to summarize, I think it's fair to say that the full-frame 12bit CinemaDNG material of the Sigma fp simply shows the benefit of a larger sensor and its lower image noise (even at base ISO if you compare the full-size TIFFs). It's thus only logical that it holds up better in extreme grades and in low light.
My likely user error in nailing the focus of Sigma fp also shows the strengths of the Pocket 4K, namely better camera assist functions and overally a better user interface/more practical user experience for video shooting. - But it's also nice to see that the Sigma fp has some genuine advantages over the Pocket 4K, especially for my own type of videomaking which revolves around event videos (concerts at indie/DIY venues) shot in extreme low light conditions.
 

Since I found the YouTube comparison between the Sigma fp and the Blackmagic Pocket Cinema Camera 4K so worthless, I did a comparison test of my own.
I wanted to compare the cameras in two respects: 
(1) dynamic range when shooting a high-contrast subject under optimal light/with optimal exposure + robustness in grading the resulting image;
(2) low light/extreme high ISO image.
My setup was as follows:
Completely dark room; Both cameras with the same lens, Tokina 28-70mm/2.8 (Nikon mount adapted to L-mount and MFT respectively) at the same aperture setting (f4) on both cameras, but at 35mm focal length on the Pocket 4K and at 70mm on the Sigma fp to compensate for the different sensor sizes; Record settings: UHD 23.98p on both cameras, full frame CinemaDNG 12bit on the Sigma fp and BRAW Q0 on the Pocket 4K (= best quality codec settings on both cameras);  For the high-contrast test: daylight-temperature LED fresnel with maximum focus/spotlight on a plastic appliance reflecting some of the light to create high contrast; both cameras at minimum/native ISOs exposed ETTR/to the right. Both cameras set to 11.25 degrees shutter angle (in lieu of an ND filter, since motion rendering is irrelevant in this test).
(Note: Lowest, respectively native, ISOs on the two cameras are ISO 100 on the Sigma fp and ISO 400 on the Pocket 4K. Never mind that nominal difference, both cameras have almost identical clipping behavior at these settings, i.e. ISO 100 on the Sigma fp behaves like ISO 400 on the Pocket 4K. On both cameras, the zebras turned out to be reliable indicators for sensor clipping: To go absolutely sure that I would optimally expose the sensor, I also shot the scene at larger shutter angles - 22.5, 45, 90 and 172.5 degrees -, with zebras popping up as early as at 22.5 on both cameras. When looking at the material in Resolve, this was indeed where clipping had occurred and waveforms remained clipped in the RGB parade even when lowering exposure in Resolve's Raw control tab.)  For the low-light test: same 'scene' as above, but with the LED fresnel turned off and only a practical light in the background turned on. Both cameras set to maximum ISO (256.000), at f4 and 172.5 degrees shutter angle. Treatment in Resolve:
Basic image adjustments only in the Raw tab, interpretation in P3 color space with Rec. 709 gamma: adjustment of white balance/tint and exposure to make the Sigma fp and Pocket 4K footage match. (The raw material of the Blackmagic Pocket 4K was much warmer than that of the Sigma fp with the same Kelvin settings...) No highlight recovery. (Wasn't necessary anyway since there was no clipping in the images.) No noise filtering or sharpening, although the Pocket 4K's BRAW codec already has some baked-in temporal noise filtering. For the extreme grade, only a solarization-like custom curve was applied that pushed the shadows to the maximum and was meant to provoke banding in the material by pushing contrasts:
I exported 16bit TIFF screengrabs which can be downloaded here (6 TIFF files in a zip archive, 133 MB).
Here's how the well-exposed high-contrast scene looks like (25% downscaled images):

Sigma fp

Pocket 4K

- Note that the difference in sharpness may be my user error, and is also influenced by the different depth-of-field between 35mm/f4 on MFT and 70mm/f4 on full frame. The manual focus aides on the Pocket 4K are much better with latest firmware, so nailing focus without an external monitor was easier.

1:1 crops of the above two images:
 
Sigma fp (left) - Pocket 4K (right).
 
Extreme grade, with the same curve (as posted above) applied to the two above images:


Sigma fp

Pocket 4K
1:1 crops of the above:

Sigma fp

Pocket 4K
 
Low light, with both cameras at maximum (256,000) ISO, 172.5 degrees shutter and f4:

Sigma fp

Pocket 4K
1:1 crops of the above:

Sigma fp

Pocket 4K
So, to summarize, I think it's fair to say that the full-frame 12bit CinemaDNG material of the Sigma fp simply shows the benefit of a larger sensor and its lower image noise (even at base ISO if you compare the full-size TIFFs). It's thus only logical that it holds up better in extreme grades and in low light.
My likely user error in nailing the focus of Sigma fp also shows the strengths of the Pocket 4K, namely better camera assist functions and overally a better user interface/more practical user experience for video shooting. - But it's also nice to see that the Sigma fp has some genuine advantages over the Pocket 4K, especially for my own type of videomaking which revolves around event videos (concerts at indie/DIY venues) shot in extreme low light conditions.
 

Since I found the YouTube comparison between the Sigma fp and the Blackmagic Pocket Cinema Camera 4K so worthless, I did a comparison test of my own.
I wanted to compare the cameras in two respects: 
(1) dynamic range when shooting a high-contrast subject under optimal light/with optimal exposure + robustness in grading the resulting image;
(2) low light/extreme high ISO image.
My setup was as follows:
Completely dark room; Both cameras with the same lens, Tokina 28-70mm/2.8 (Nikon mount adapted to L-mount and MFT respectively) at the same aperture setting (f4) on both cameras, but at 35mm focal length on the Pocket 4K and at 70mm on the Sigma fp to compensate for the different sensor sizes; Record settings: UHD 23.98p on both cameras, full frame CinemaDNG 12bit on the Sigma fp and BRAW Q0 on the Pocket 4K (= best quality codec settings on both cameras);  For the high-contrast test: daylight-temperature LED fresnel with maximum focus/spotlight on a plastic appliance reflecting some of the light to create high contrast; both cameras at minimum/native ISOs exposed ETTR/to the right. Both cameras set to 11.25 degrees shutter angle (in lieu of an ND filter, since motion rendering is irrelevant in this test).
(Note: Lowest, respectively native, ISOs on the two cameras are ISO 100 on the Sigma fp and ISO 400 on the Pocket 4K. Never mind that nominal difference, both cameras have almost identical clipping behavior at these settings, i.e. ISO 100 on the Sigma fp behaves like ISO 400 on the Pocket 4K. On both cameras, the zebras turned out to be reliable indicators for sensor clipping: To go absolutely sure that I would optimally expose the sensor, I also shot the scene at larger shutter angles - 22.5, 45, 90 and 172.5 degrees -, with zebras popping up as early as at 22.5 on both cameras. When looking at the material in Resolve, this was indeed where clipping had occurred and waveforms remained clipped in the RGB parade even when lowering exposure in Resolve's Raw control tab.)  For the low-light test: same 'scene' as above, but with the LED fresnel turned off and only a practical light in the background turned on. Both cameras set to maximum ISO (256.000), at f4 and 172.5 degrees shutter angle. Treatment in Resolve:
Basic image adjustments only in the Raw tab, interpretation in P3 color space with Rec. 709 gamma: adjustment of white balance/tint and exposure to make the Sigma fp and Pocket 4K footage match. (The raw material of the Blackmagic Pocket 4K was much warmer than that of the Sigma fp with the same Kelvin settings...) No highlight recovery. (Wasn't necessary anyway since there was no clipping in the images.) No noise filtering or sharpening, although the Pocket 4K's BRAW codec already has some baked-in temporal noise filtering. For the extreme grade, only a solarization-like custom curve was applied that pushed the shadows to the maximum and was meant to provoke banding in the material by pushing contrasts:
I exported 16bit TIFF screengrabs which can be downloaded here (6 TIFF files in a zip archive, 133 MB).
Here's how the well-exposed high-contrast scene looks like (25% downscaled images):

Sigma fp

Pocket 4K

- Note that the difference in sharpness may be my user error, and is also influenced by the different depth-of-field between 35mm/f4 on MFT and 70mm/f4 on full frame. The manual focus aides on the Pocket 4K are much better with latest firmware, so nailing focus without an external monitor was easier.

1:1 crops of the above two images:
 
Sigma fp (left) - Pocket 4K (right).
 
Extreme grade, with the same curve (as posted above) applied to the two above images:


Sigma fp

Pocket 4K
1:1 crops of the above:

Sigma fp

Pocket 4K
 
Low light, with both cameras at maximum (256,000) ISO, 172.5 degrees shutter and f4:

Sigma fp

Pocket 4K
1:1 crops of the above:

Sigma fp

Pocket 4K
So, to summarize, I think it's fair to say that the full-frame 12bit CinemaDNG material of the Sigma fp simply shows the benefit of a larger sensor and its lower image noise (even at base ISO if you compare the full-size TIFFs). It's thus only logical that it holds up better in extreme grades and in low light.
My likely user error in nailing the focus of Sigma fp also shows the strengths of the Pocket 4K, namely better camera assist functions and overally a better user interface/more practical user experience for video shooting. - But it's also nice to see that the Sigma fp has some genuine advantages over the Pocket 4K, especially for my own type of videomaking which revolves around event videos (concerts at indie/DIY venues) shot in extreme low light conditions.
 

Since I found the YouTube comparison between the Sigma fp and the Blackmagic Pocket Cinema Camera 4K so worthless, I did a comparison test of my own.
I wanted to compare the cameras in two respects: 
(1) dynamic range when shooting a high-contrast subject under optimal light/with optimal exposure + robustness in grading the resulting image;
(2) low light/extreme high ISO image.
My setup was as follows:
Completely dark room; Both cameras with the same lens, Tokina 28-70mm/2.8 (Nikon mount adapted to L-mount and MFT respectively) at the same aperture setting (f4) on both cameras, but at 35mm focal length on the Pocket 4K and at 70mm on the Sigma fp to compensate for the different sensor sizes; Record settings: UHD 23.98p on both cameras, full frame CinemaDNG 12bit on the Sigma fp and BRAW Q0 on the Pocket 4K (= best quality codec settings on both cameras);  For the high-contrast test: daylight-temperature LED fresnel with maximum focus/spotlight on a plastic appliance reflecting some of the light to create high contrast; both cameras at minimum/native ISOs exposed ETTR/to the right. Both cameras set to 11.25 degrees shutter angle (in lieu of an ND filter, since motion rendering is irrelevant in this test).
(Note: Lowest, respectively native, ISOs on the two cameras are ISO 100 on the Sigma fp and ISO 400 on the Pocket 4K. Never mind that nominal difference, both cameras have almost identical clipping behavior at these settings, i.e. ISO 100 on the Sigma fp behaves like ISO 400 on the Pocket 4K. On both cameras, the zebras turned out to be reliable indicators for sensor clipping: To go absolutely sure that I would optimally expose the sensor, I also shot the scene at larger shutter angles - 22.5, 45, 90 and 172.5 degrees -, with zebras popping up as early as at 22.5 on both cameras. When looking at the material in Resolve, this was indeed where clipping had occurred and waveforms remained clipped in the RGB parade even when lowering exposure in Resolve's Raw control tab.)  For the low-light test: same 'scene' as above, but with the LED fresnel turned off and only a practical light in the background turned on. Both cameras set to maximum ISO (256.000), at f4 and 172.5 degrees shutter angle. Treatment in Resolve:
Basic image adjustments only in the Raw tab, interpretation in P3 color space with Rec. 709 gamma: adjustment of white balance/tint and exposure to make the Sigma fp and Pocket 4K footage match. (The raw material of the Blackmagic Pocket 4K was much warmer than that of the Sigma fp with the same Kelvin settings...) No highlight recovery. (Wasn't necessary anyway since there was no clipping in the images.) No noise filtering or sharpening, although the Pocket 4K's BRAW codec already has some baked-in temporal noise filtering. For the extreme grade, only a solarization-like custom curve was applied that pushed the shadows to the maximum and was meant to provoke banding in the material by pushing contrasts:
I exported 16bit TIFF screengrabs which can be downloaded here (6 TIFF files in a zip archive, 133 MB).
Here's how the well-exposed high-contrast scene looks like (25% downscaled images):

Sigma fp

Pocket 4K

- Note that the difference in sharpness may be my user error, and is also influenced by the different depth-of-field between 35mm/f4 on MFT and 70mm/f4 on full frame. The manual focus aides on the Pocket 4K are much better with latest firmware, so nailing focus without an external monitor was easier.

1:1 crops of the above two images:
 
Sigma fp (left) - Pocket 4K (right).
 
Extreme grade, with the same curve (as posted above) applied to the two above images:


Sigma fp

Pocket 4K
1:1 crops of the above:

Sigma fp

Pocket 4K
 
Low light, with both cameras at maximum (256,000) ISO, 172.5 degrees shutter and f4:

Sigma fp

Pocket 4K
1:1 crops of the above:

Sigma fp

Pocket 4K
So, to summarize, I think it's fair to say that the full-frame 12bit CinemaDNG material of the Sigma fp simply shows the benefit of a larger sensor and its lower image noise (even at base ISO if you compare the full-size TIFFs). It's thus only logical that it holds up better in extreme grades and in low light.
My likely user error in nailing the focus of Sigma fp also shows the strengths of the Pocket 4K, namely better camera assist functions and overally a better user interface/more practical user experience for video shooting. - But it's also nice to see that the Sigma fp has some genuine advantages over the Pocket 4K, especially for my own type of videomaking which revolves around event videos (concerts at indie/DIY venues) shot in extreme low light conditions.
 

The A7iii is a good allround video camera and really excellent stills camera (I do own it), but no cinema camera because of its weak codecs and 8bit limitation, no matter how you rig it up.

The A7iii is a good allround video camera and really excellent stills camera (I do own it), but no cinema camera because of its weak codecs and 8bit limitation, no matter how you rig it up.

The A7iii is a good allround video camera and really excellent stills camera (I do own it), but no cinema camera because of its weak codecs and 8bit limitation, no matter how you rig it up.

The A7iii is a good allround video camera and really excellent stills camera (I do own it), but no cinema camera because of its weak codecs and 8bit limitation, no matter how you rig it up.

The A7iii is a good allround video camera and really excellent stills camera (I do own it), but no cinema camera because of its weak codecs and 8bit limitation, no matter how you rig it up.

The A7iii is a good allround video camera and really excellent stills camera (I do own it), but no cinema camera because of its weak codecs and 8bit limitation, no matter how you rig it up.

The A7iii is a good allround video camera and really excellent stills camera (I do own it), but no cinema camera because of its weak codecs and 8bit limitation, no matter how you rig it up.

Just to compare:
In 2009, a DIY filmmaker equipped with the newest gear would have used
- a Canon 5D MkII or 7D with the mushy h264 image whose actual optical resolution was closer to SD than HD, in sRGB color profiles with a dynamic range of 7-8 stops;
- on a steady cam like the Glidecam, or a bulky DSLR rig with display loupes;
- with adapted vintage manual DSLR lenses because of the nearly unusable manual focus of native AF lenses (and AF still being unsuitable for video);
- editing the material in Final Cut Pro 7 or Premiere Pro using Magic Bullet Looks for color grading, with material that didn't really have enough bit depth for doing that.

Just to compare:
In 2009, a DIY filmmaker equipped with the newest gear would have used
- a Canon 5D MkII or 7D with the mushy h264 image whose actual optical resolution was closer to SD than HD, in sRGB color profiles with a dynamic range of 7-8 stops;
- on a steady cam like the Glidecam, or a bulky DSLR rig with display loupes;
- with adapted vintage manual DSLR lenses because of the nearly unusable manual focus of native AF lenses (and AF still being unsuitable for video);
- editing the material in Final Cut Pro 7 or Premiere Pro using Magic Bullet Looks for color grading, with material that didn't really have enough bit depth for doing that.

- Panasonic GH2 (released in 2010)
- MagicLantern RAW on 5D MKii + MKiii + EOS-M, the Blackmagic Cinema Camera 2.5K (still incredibly good) + all versions of the Blackmagic Pocket (& Micro) Cinema Camera
- Metabones Speed Booster
- Gimbals (Zhiyun, DJI, Moza) including the DJI Osmo Pocket gimbal camera
- Sigma Art lenses, all of them, but especially the 18-35mm/1.8
- Davinci Resolve and (not necessarily in my niche) FCPX
All these were real game changers.

- Panasonic GH2 (released in 2010)
- MagicLantern RAW on 5D MKii + MKiii + EOS-M, the Blackmagic Cinema Camera 2.5K (still incredibly good) + all versions of the Blackmagic Pocket (& Micro) Cinema Camera
- Metabones Speed Booster
- Gimbals (Zhiyun, DJI, Moza) including the DJI Osmo Pocket gimbal camera
- Sigma Art lenses, all of them, but especially the 18-35mm/1.8
- Davinci Resolve and (not necessarily in my niche) FCPX
All these were real game changers.

Yeah, considered selling mine as well.
While it's a decent-enough mini RAW video camera (except for the file sizes of uncompressed CinemaDNG), it's a lousy performer as a stills camera - most annoyingly lacking responsiveness, with the abysmal autofocus and 0.1 seconds shutter delay.
I've remorsefully gone back to my Sony A7iii for stills shooting, a much better camera in that department....

For me, the shutter delay currently is a deal breaker for using the camera for serious, daily photography.
You can easily test it: Turn 'Shutter Blackout' off in the camera menu (section 'Shoot', 5th tab), shoot a digital stopwatch like this one, and you will see that the actually recorded picture has been taken 0.1 seconds later than the picture/frame shown (and briefly frozen) in the camera display after pressing the shutter button.
It literally means that you can't take snapshots of moving subjects with the camera, at least not in a predictable way. Unreliable focus and inconsistent metering make this a camera with an unusually high amount of pictures you have to throw away in post because of missed moments, missed focus and wrong exposure. 
I have to second DPreview's opinion that the fp is one of the worst cameras released in 2019, from a stills photography perspective. It's most likely the worst full-frame photography camera currently on the market, maybe even the worst full frame digital photography camera ever released.
In fact, a $250 EOS-M100 (with its similar-sized body and a pancake lens like the Canon 22mm/f2) beats it as a stills camera, except for the sensor size. Since I had both cameras, I know what I'm talking about... [and would already regret having given the EOS-M100 to my girlfriend after I bought the Sigma - if it hadn't been for my girlfriend...]

You can try renaming the folder and dig files to the structure of the following screenshot. 
This is the resolve naming convention.