Good news for Apple TV users, the device can now play YouTube videos in 4K. But there are a few limitations. You can only watch videos in Ultra HD at 30 frames per second and without HDR. If you want to take advantage of 60fps videos, these can play at up to 1440p. 4K support is showing up for users who have at least tvOS 14 running on their media hub. YouTube appears to be rolling out the update remotely, and it is doing it gradually, so don't be surprised if you haven't gotten it yet.
YouTube also confirmed that 4K video with 60fps and HDR is available on the current iPads and iPhones, too. But Engadget pointed out that support seems to be inconsistent right now.