Selected Xbox 360 games are Xbox One X Enhanced, leveraging the additional power of the console for higher resolution, 9 times the original pixel count, and expanded colour details. Since the release of the Xbox One S, Xbox owners have questioned when the app would be upgraded to support the buzz around 4K, though it certainly took quite a while for the change to be made.
It definitely took a while for the app to receive 4K support on the Xbox One, but what matters is that it's here now. And if you aren't tech savvy and thought that only the Xbox One X supported 4K, that is just for gaming.
Users should start seeing their YouTube app automatically update shortly, though those who are impatient can head to the Microsoft Store to speed up the process.