4K movies from iTunes can be streamed but not downloaded locally
You can rent or purchase 4K movies from iTunes Store, but you cannot download them locally to your devices like with 1080p (HD) and standard definition (SD) content offered on the store.
BOTTOM LINE: new 4K purchases and previous HD purchases upgraded to 4K are always available to you for streaming in SD/HD/4K, but you can download them locally only in HD/SD.
According to Apple’s support document, you can download a local copy of an HD or SD movie from iTunes to your iPhone, iPad, Mac or PC just like you’ve always been able to (Apple TV intelligently caches purchased movies, depending on available storage).
You cannot, however, download a 4K version of the movie onto your Apple ID-authorized devices. This puts a new definition on “renting” and “purchasing” so here’s a quick cheat sheet:
BUY SD/HD: The movie can be downloaded locally onto your iOS devices and computers to keep forever and streamed unlimitedly on Apple TV.
RENT SD/HD: The movie is cached locally on your iOS devices, computers and Apple TVs for smooth playback and automatically deleted after 24 hours in the United States and 48 hours everywhere else. You have 30 days to start watching your rental. The OS automatically plays the best quality of video available for your device and Internet connection.
BUY 4K: The movie can be streamed unlimitedly in SD/HD/4K on your iOS devices, computers and Apple TVs. You can also download it locally onto your iOS devices and computers in SD/HD resolution only.
RENT 4K: The movie is always streamed to your iOS devices, computers and Apple TVs and cannot be downloaded to keep the file forever. After you begin streaming, you have 24 hours to watch your 4K rental in the United States and 48 hours to watch it everywhere else. You have 30 days to start watching your rental. The OS automatically plays the best quality of video available for your device and Internet connection.
On the surface, this sounds like another annoying limitation.
In reality, however, this isn’t such a big deal as it sounds because you’re getting 4K versions of iTunes movies essentially for free if you already own them in HD or at the same asking price of the latest and greatest flicks, which is typically set at $19.99.
Besides, all video content carried on iTunes Store has always been DRM-protected using Apple’s proprietary FairPlay mechanism.
FairPlay limits your iTunes movies to playback on iPhone, iPad, Mac or PC devices authorized with the same Apple ID account you’ve used to buy them.
Apple is upgrading all of our previous HD movie purchases to 4K versions, where available, completely free of charge—which is unheard of—so I’m guessing this limitation is some sort of concession to content owners in exchange for free HD→4K upgrades.
I doubt this new policy indicates a licensing issue.
More likely, Hollywood majors are afraid that someone might beat FairPlay encryption so they’ve probably imposed this limitation upon Apple.
http://www.idownloadblog.com/2017/09/21/4k-movies-itunes-no-download/