Just go to the Play store in a desktop or laptop and search for Ariel Dream. It adds the Apple Tv video screensavers to the day dream option and they look stunning on a 4k tv :)

Ariel Dream.

However I always had problems with it on my MiBox after a couple of runs. Annoying popup saying it failed to load would stay forever, ended up uninstalling it. Maybe you'll have better luck!