However, some HD files will still include HDR and Dolby Vision. Apple says, "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." And if your Internet speed isn't fast enough, the quality of video will be downgraded to what your connection can support.
When you think about space limitations for 4K files, it does make sense. Ultra HD files are pretty big and will take up a lot of space on an iPhone or iPad. Additionally, the Apple TV 4K is the only device that can play these files right now. But at the same time, some people like downloading their movies and streaming locally, off of a computer, rather than relying on a external Internet connection. There may be some logic behind this decision, but that doesn't mean it isn't a frustrating discovery. Still, it's a free upgrade, so it's difficult to complain too much.
Update: Apple has confirmed that you will only be able to stream upgraded 4K movies, but as we pointed out, you wouldn't want Ultra HD downloads on an iPad or iPhone. They're looking into the situation for 4K Mac downloads and we'll update when we have more.