Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
Just tried my Ultra (model 4640) with 4K HDR videos from YouTube and had no issue. The TV is a Panasonic ST30 1080 plasma, so pretty much the same scenario as yours. I have seen a green screen in the past, and it was a bad HDMI cable.
Dan Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.
Okay now videos on Youtube marked 4K display fine.
I don't know why.
My only guess is a restart of the Roku I did a couple of days ago because it seemed to be acting sluggish might have fixed it (the sluggishness actually turned out to be a PC problem. My PC running my Plex server was bogged down and wasn't able to stream videos and I thought it was the Roku).
I tried another 4K video today and it displayed fine. I then went back to the other 4K videos that I had previously had display issues with and they played fine!.
A head scratcher for sure but at least it has a happy ending!