https://www.theverge.com/2017/9/22/16349496/apple-tv-4k-itunes-streaming-only-no-download

The Apple TV 4K hits stores today, but it might be helpful to know that, while the device will finally support 4K content, you’ll only be able to stream said content, not download it locally. MacRumors reports that a support document from Apple regarding the new set-top device clearly states that, while you can download HD content from iTunes, 4K is strictly for streaming. It’s unclear if that will also apply to HDR and Dolby Vision versions.

“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 document reads. If you already bought HD content from iTunes and want that free upgrade to 4K, you will still only be able to watch offline in HD. In the same support document, Apple recommends an internet speed of 25Mbps for 4K streaming, or it will automatically switch down to a lower resolution.