If you've gotten on board with the Xbox One S, you can stream Amazon's original shows in 4K on the Amazon Prime app as of today.
While that seems like a pretty limited selection (and honestly, it is), it's still a good sign of the times. 4K is the new HD, and it's nice to see content providers gearing up for when everyone has 4K TVs. Netflix has been streaming 4K for a while now, so Amazon joining the team is a good omen for the future.
You can read the official Xbox post here.