A High-Speed Internet Connection4K streaming requires significant bandwidth. Apple recommends a minimum sustained download speed of 25 Mbps. If your connection drops below this threshold, the Apple TV app will automatically downscale the video quality to 1080p to prevent buffering.
To experience 4K resolution from your iTunes library, you must stream the content over a high-speed internet connection. How to Stream iTunes Movies in 4K download 4k movies from itunes
The short answer is no, you cannot download a local copy of a 4K movie to a computer or a mobile device to watch offline in full 2160p resolution. download 4k movies from itunes