just films/TV via iTunes? Is there any difference app wise compared to ATV4?
One of the top sources of 4k content is Netflix. The vast majority of their shows and own movies are UHD. If it wasn't for them my 4k consumption on the Apple TV would be minimal. YouTube on ATV is 1080p only which is hurting because I watch a lot of YouTube.
Amazon Prime have a films and shows in 4k. iTunes has 4k movies but it's not guaranteed for new movies and sometimes rental isn't an option which means that you have to waste a lot of money buying movies that you might not be good enough to watch a second time.
Vimeo also have 4k content if you like short independent movies, reels or documentaries.