I had the exact same problem with the Apple TV 4k (OS 11.1, most recent update)
TV: Hisense 55H9B2 4k curved tv
Cable: Apple-recommended Belkin Ultra HDMI cable
(plus about 6 other 18Gbps cables I bought from Monoprice for much less)
All YouTube 4k content played fine, and I could play 4k movies from iTunes with no problems. Netflix would allow me to browse the content, but when I played 4k videos I would hear sound and could see the UI elements but the video stream was a green screen. When I played regular 1080P content the screen was black, but I could hear sound and see UI.
The Apple TV 4k automatically chose 4k SDR, and would not switch to any other output resolutions. I called Netflix and Apple trying to troubleshoot, but neither could find a solution except to point out that it seems like an HDCP 2.2 issue and to try upgrading my cable. I tried 7 difference cables, including the recommended one from Belkin.
I finally found a post on Amazon for my TV which mentioned that my TV firmware needed an update. The TV's online firmware update feature returned nothing, but I knew mine was the oldest shipped for this TV. Thankfully, someone posted the most recent firmware file on Amazon. I updated via flash drive, and suddenly the Apple TV 4k asked to upgrade the display output to 4k 60Hz HDR. After I changed the output, Netflix started displaying all videos properly.
Check for a firmware update for your television.
For reference for anyone with the Hisense 55H9B2 TV, I upgraded from F1211 to H0117 in order to enable 4k 60Hz HDR (https://www.amazon.com/gp/customer-reviews/R24J42DVDCE5FM/ref=cm_cr_getr_d_rvw_t tl?ie=UTF8&ASIN=B00ZGIJDOU).