Julian's images: saving the 4K example at quality 6 creates DCT macroblock artifacts that don't show up in the 444 example at quality 10. All the images posted are 420: that's JPG. To compare the 1080p 444 example to the 4K 420 example: bicubic scale up the 1080p image to match exactly the same image region as the 4K image (examples posted are different regions and scale). The 1080p image will be slightly softer but should have less noise and artifacts. Combining both images as layers in a image editor then computing the difference (and scaling the brightness/gamma) up so the changes are clearly visible will help show exactly what has happened numerically; helpful if the differences aren't very obvious on visual inspection.
We agree that 420 4K scaled to 1080p 444 will look better than 1080p captured at 420 (need to shoot a scene with camera on tripod and compare A/B to really see benefits clearly). 444 has full color sampling per pixel vs 420 having 1/4 the color sampling (1/2 vertical and 1/2 horizontal). My point is that we're not really getting any significant color element bit depth improvement which allows significant post-grading latitude as provided by a native 10-bit capture (at best there's ~8.5-9-bits of information encoded after this process: will be hard to see much difference when viewed normally (vs. via analysis)). Another thing to keep in mind is that all > 8-bit (24-bit), e.g. 10-bit (30-bit) images, need a 10-bit graphics card and monitor to view. Very few folks have 10-bit systems (I have a 10-bit graphics card in one of my machines, but am using 8-bit displays). >8-bit systems images need to be dithered and/or tone mapped to 8-bit to take advantage of the >8-bit information. Everything currently viewable on the internet is 8-bit (24-bit) and almost all 420 (JPG and H.264).
re: H.264 being less that 8-bits- it's a effectively a lot less than 8-bits not only from initial DCT quantization and compression (for the macroblocks), but also from the motion vector estimation, motion compression, and macro block reconstruction (which includes fixing the macroblock edges on higher quality decoders).