The device Apple TV 4K It already starts playing YouTube videos in 4K. And I say that it "starts" playing, because only some users have started to notice it on their devices. It is not a question of updating the YouTube app and having it fixed in a generalized way, but rather of the streaming video company "updating" its servers.
So if you are lucky enough to connect to a server YouTube update, you will see the option to play a 4k video in its maximum resolution, if your device is an Apple TV 4K, of course. If it does not appear, you will have to wait for the YouTube server that provides you with said video to be able to do so.
When Apple released a few days ago TvOS 14, one of its "official" news announced by the company was that it could play YouTube videos in 4K on devices compatible with Ultra Definition. Since here We already reported that this novelty did not work, without being very clear if it was the fault of Apple or YouTube.
Now we know that the culprit is YouTube, since without having to do anything on the devices, some users are already starting to have this function activated. It seems that YouTube's servers have to be "updated" to be able to offer this service, and some are already doing so. That is why implantation is not general, but selective.
Lucky users who already have this feature, comment that if they choose 4k playback, it is done only at 30 fps. Those that are reproduced at 50 or 60 fps, have a maximum resolution of 1440p. They also report that at the moment nothing in HDR mode.
So for now we will have to wait to see how YouTube is implementing the possibility of 4K playback on Apple TV 4K devices, and if they will reach 60 fps, and quality HDR. We will see.