The new 4K Apple TV will convert your existing HD titles from iTunes to a 4K resolution, but the 4K movies will not be stored to the device.
This is a crucial detail Apple avoided to unveil when it advertised the new set top box.
So Apple will let you watch 4K versions of your HD movies, but that doesn't mean you can download them locally and play them whenever you feel like it. 4K content can only be streamed using the Apple TV 4K.
"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," Apple says.
This means that if you don't have a minimum internet speed of 25Mbps, then you won't be able to stream content in 4K either.