The new 4K content available in the iTunes Store can't be downloaded, only streamed, according to an Apple support document.
Apple notes that, "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."
This is disappointing news for those who want to watch 4K content offline or have a slow connection and are unable to stream at the required 25 Mbps for 4K streaming.