Besides Netflix and movies from iTunes, you can also stream 4K movies from your NAS. I use Infuse which supports 4K and HDR (no support for Dolby Vision yet).
Only "problem" with Apple TV 4K is the lack of support for Youtube 4K HDR. If I'm not mistaken, it plays Youtube in 1080p only which is a shame.
This post has been edited by welwitchia: Jul 16 2018, 02:30 PM
(2017) Apple TV 4K Discussion Thread, Faster gpu than new 2021 Apple TV