Origine :
Il contenuto 4K può essere solo trasmesso in streaming e non scaricato.
Per Apple:
When you buy or rent a movie from the iTunes Store, it automatically plays in the highest-quality format available for the device on which you're watching it. You can download a local copy of an HD movie to your iPhone, iPad, iPod touch, Mac, or PC, and you might be able to download HDR and Dolby Vision versions to your iOS devices, but you can't download a 4K version.
To see if a movie can play in 4K on a compatible device, look for 4K icon on the product page. By default, all videos that you can buy or rent in the iTunes Store are available to play back in HD3.
Movies in the iTunes Store might also be available in HDR10 HDR icon or Dolby Vision Dolby Vision icon. Content automatically plays in the HDR format that is compatible with your device.
Potrebbe essere possibile che il film sia disponibile in 4K ma il dispositivo che stavi utilizzando per navigare su iTunes non era in grado di visualizzare 4K? In questo caso, non credo iTunes mostrerebbe la versione 4K.