The new Apple TV 4K is now available to buy, the device went on sale last Friday. This is Apple’s first 4K compatible set top box, although it would appear that the device does not support YouTube 4K videos.
Apple has not released any information on why 4K YouTube videos are not supported on their new Apple TV, although there is a lot of speculation on why these videos are not supported.
The new Apple TV supports the H.264/HEVC codec and Google is apparently using the VP9 codec, which Apple does not support.
This is apparently the reason why YouTube videos in 4K are not supported on the new Apple TV, it is not clear as yet on whether this will change in the future. Apple could decide to add support for 4K YouTube videos some time in the future, although whether they will do this remains to be seen.