http://www.techradar.com/news/youtube-finally-allows-4k-streaming-on-xbox-one

The ability to play media in 4K is one of the big selling points of both the Xbox One S and the Xbox One X, but until now, you haven't been able to take advantage of that feature with YouTube on the devices without resorting to third-party apps like myTube.

That changes today with the launch of a new, separate YouTube app for both consoles that lets you watch YouTube videos in both glorious 4K resolution and at 60 frames per second. Curiously, though, the app doesn't let you watch videos in HDR. You win some, you lose some.