As far as I know Apple doesn't currently offer 4K content and the Apple TV doesn't have hardware capable of outputting 4K. You'll need Sony's 4K streaming device if you want to watch 4K content. Currently its really the only device with a decent set of 4K videos and movies/TV shows worth watching. There are other 4K streaming devices but none of them seem to have access to quality content worth watching. Alternatively you can buy or build a PC if you want to watch 4K clips from YouTube or video generated from consumer video recorders (GoPro's or Drones for example).