http://www.techradar.com/news/you-cant-download-4k-content-on-the-new-apple-tv-but-rental-times-are-up

Something to note if you're rushing out to buy a new Apple TV 4K this weekend: the box can only stream content in 4K rather than downloading it and playing it offline, as per an official support document published by Apple.

That's perhaps not a huge surprise considering the limited storage on the Apple TV and the huge size of 4K films and television shows, but it does mean you should double-check the speed of your internet connection to make sure it can cope with all those pixels. Apple recommends a minimum speed of 25 Mbps for 4K streaming.