https://arstechnica.com/gadgets/2017/09/itunes-does-not-support-downloading-4k-content-only-streaming/

Apple announced the arrival of more 4K content to iTunes when it introduced the new Apple TV 4K last week. However, those who purchase the new set-top box will be limited in how they can enjoy 4K titles. According to an Apple support document, 4K content from iTunes can only be streamed, not downloaded directly to a device.

"You can download a local copy of an HD movie, and you might be able to download HDR and Dolby Vision versions, but you can't download a 4K version," the support document states.