Movies in 4K resolution from iTunes you can only stream but not download
Apple updated the set of rules that relate to video content presented in iTunes. The update was made in connection with the release of Apple TV 4K, which is now sold.
Apple notes that to stream movies in 4K resolution from iTunes you will need a high speed (25 Mbps) Internet connection . If the speed is less, the quality of the video stream will be reduced.
It is also noted that now download movie you in HD resolution. In some cases it is possible to download movies that support HDR and Dolby Vision. However, movies in 4K resolution to download for offline browsing is not allowed, only streaming.
The reason for this decision by Apple not made public.