Here's a very nicely done video from Paul Shillito at Video-Alchemy, who gives us an in-depth discussion of keying with 10-bit ProRes 422 and uncompressed RAW video from the 2.5K Blackmagic Cinema Camera (BMCC) - and then compares the results to an 8-bit 4:2:0 compressed video key from the 4K Panasonic GH4.
In this 15:25 video, Paul does a 9 minute, detailed walk through of the setup - and then about 6 minutes of side-by-side conclusions. If you'd like to cut straight to the bottom line, please click here.
It would be reasonable to expect that the 8-bit compressed video would not hold up well to the key as uncompressed RAW - but let's take a look at the video and see:
So - to my eyes at least, compressed 4K looks clean enough for a production quality key. This should be interesting to two groups of people. First, those who have already bought their GH4s, and who have had concerns that they'll need to buy an external 10-bit recorder to get a clean key. Those folks can relax - internally recorded 4K works just fine.
The second group is those who have hesitated before buying the GH4 because they were concerned about this issue. Those folks can relax too - and go ahead and order their cameras. The best price for an in-stock GH4 right now is $1793.95 at Cameta via eBay.