While 4K films on iTunes are streaming-only, it appears that users may be able to upgrade their movie collection if they buy a physical Blu-ray copy that comes with an iTunes Digital Copy code, even if the Blu-ray is just 1080p HD.
Upon setting up the new Apple TV 4K, AppleInsider checked iTunes movie libraries and found that recently redeemed iTunes Digital Copy films are listed in the iTunes movie library as being 4K-resolution. This despite the fact that the Blu-rays in question were not 4K versions.
Notably, years ago iTunes Digital Copy redemptions only offered standard-definition versions of films via Apple's store when redeemed.
Such legacy redemptions have never been upgraded to high definition, let alone 4K. That remains the case with the Apple TV 4K -- older iTunes Digital Copy films remained available to stream in standard definition, even though HD copies are available to purchase on the iTunes store.
To that end, when accessed via the Apple TV, the iTunes library items do not show an HD icon.
But when loading up a digitally redeemed copy of the film "Logan" from 20th Century Fox, the copy owned in the iTunes library shows that it is, in fact, available for streaming 4K, as well as HD.
Unfortunately, it's impossible to tell whether or not the film itself is actually streaming in 4K quality, as tvOS on the Apple TV runs at 4K resolution at all times, regardless of the quality of the content being played.
Further, although streamed programming may be available at resolutions up to 4K, the ability to reach that high of quality is largely dependent on the user's home internet connection.
Still, the appearance of a 4K icon on an iTunes Digital Copy obtained from a non-4K Blu-ray would strongly suggest that users can actually stream higher quality versions of films than were purchased on a physical disc. That's noteworthy because 4K Blu-rays not only require special players, but the discs themselves also carry a premium over standard HD Blu-rays, sometimes $10 to $15 more.
Given that Apple worked out a deal with studios to charge the same $20 price for films, whether they are 4K or HD, it would appear that iTunes will treat both 4K and HD the same -- as long as streaming is concerned. Downloads, however, remain limited to 1080p HD.