For green screen resolution is very important. So, you better key in 4K and later downscale for delivery.

Why you need external recorder?

Thank you very much for both your replies and advice. I am a total noob as far as green screen goes. I haven't bought a GH4 yet I just wondered how good it would be for greenscreen. I was under the impression that an external recorder might mean it could output at a higher bit rate that would enable better keying (probably totally misunderstood that bit). If i just wanted the camera for greenscreen I guess the blackmagic option would be best but I need something a little more multi-purpose.

Vitliy You know RAW is better for VFX and green screen YES its heavier but its better.

No tests, but actual work. Not allowed to share unfortunately. And GH4's 4K modes are 100 Mbps, which isn't really that high bitrate considering the resolution. If we split it in 4 (the same as 1080/2k) then it's 25 Mbps. Regular h264.

10bit 422 1080 should key great. Mind you, if your green screen is poorly lit, all resolution and color space arguments don't amount to much, imho

422 color isn't ideal but it's doable, with proper filtering, and much better than any compressed format. 420 compressed, fuggetaboutit, that's amateur and will take a lot of effort to get decent results with. It's only advisable if you have no alternative. You'll spend effort just trying to get a decent, clean key, effort that could be spent on actually making the overall composite better.

To answer the OP, no, the GH4 is not as good as or better than any Blackmagic camera for green screen, regardless of mode, Prores or RAW, but RAW is the way to go.

edit: on top of shooting RAW, or at least 422, lens is going to be very critical. Even shooting RAW you could still get problematic edges if your lens suffers from chromatic effects. Here's where having a bland, clinical, "too sharp" lens, like, for instance, something Leica, really comes in handy. Something really modern will be better for shooting green screen than your classic lenses good for narrative and normal cinematography.

You debayer the RAW and work high precision linear.

422 Prores will still require special handling and filtering.

The only thing well thought out, regarding 4K 420, is knowing it's not good, not advisable and only something you do if you have no other choice. It's a "make do" sort of deal, making the best of a bad situation.

All of the problems with 420 1080P video are plainly evident in 420 4K imagery from the GH4. In fact the codec is even more harsh in a lot of sample imagery posted here and elsewhere than high-bit GH2 footage (highlights, particularly the blue channel). Scaling down 4K has already proven to retain macroblocking and other artifacts. There's no free lunch here. Dark, noisy edges at 4K are still going to look bad at 1080P the same way dark, bad edges done to 2K film scanned imagery still looks bad when viewed at 480P or even worse, once transferred to VHS.

All those bad edges from opticals done in the '80s at ILM were effectively much higher than 4K resolution, given they did optical work with VistaVision plates. Even after all filtering from duplication and reduction for eventual broadcast or home video release there was absolutely no improvement to their quality. A similar principle applies here.

I made a test using the GH3 internally recorded. Just guessing the GH4 externally recorded at 10 bits would be even better. If this can be achieved with a couple of clicks in Premiere I'm certain with a little love you could get good results in after effects. Check out all the detail that was kept in the fly away hairs by the crown. Didn't bother softening the key or messing with too many details but I really don't see why getting a decent key would be an issue. I've even done full body shots with green spill everywhere and keying it was no problem. It's just a matter of knowing your way around After Effects. As long as there is a difference in color there is a key to be had.

I think today's powerful keyer has made keying 4.2.0 footage much easier.

@CarlosMorillo The hair is not too bad but I still see clearly green edges at the hands and the forearms

I been doing digital keying since the video cube ;) the most important things to remember are object isolation against the background to avoid the spill, separate illuminations to the background and the subject, avoid shadow casting and keep a plain illumination on the background. Modern software make to easy to do Chroma Key, mainly digital compositing software (After effects, Nuke, etc..) thanks to better tools to do this. The image from the GH4 with a external recorder at 10Bit definitive will help, thanks to the improved resolution and additional color space, RAW data i don't think it will help too much if the first rules are not accomplished. My two cents.

@CarlosMorillo awesome for 4:2:0, corrected result was worth the effort. Well done!