OK, not sure if this is the case, but earlier today I saw on B&H's website a note re: the YAGH unit mentioning that HDMI out is disabled if the GH4 is mounted on the YAGH unit.
Therefore, if HDMI-out is disabled when using the YAGH unit, and internal recording to an SDHC/SDXC memory card is not possible when outputting 4:2:2 10-bit, am I understanding correctly then that in order to output and record 4:2:2 10-bit footage with sound from the YAGH unit’s XLR inputs that the only way to do so is to use an external recorder that works with the Quad-Link SDI Outputs, and either use the built-in OLED swivel screen or a BNC-compatible viewfinder/external monitor to see what it is I’m recording? Or will the HDMI out on the YAGH still work when the GH4 is mounted on it, and it's only the mini-HDMI out on the GH4 itself that is disabled when the camera is connected to the YAGH unit?
Note: Looking at the B&H website again just now, I no longer see the note mentioning HDMI-out being disabled when the camera is on the YAGH unit, so perhaps they pulled it down because it was incorrect?
UPDATE: I've just read that it's only the mini-HDMI out on the camera itself that is disabled when the GH4 is attached to the YAGH unit, and that the YAGH unit's HDMI out is not disabled so I have the answer I was seeking. Please disregard this post.