Ah, Apple... you sneaky devils. When the company announced their new 4K-capable Apple TV, they left out an important fact: you can't download 4K movies to watch them later, you have to stream them.
Apple will let you watch 4K versions of your movies on the new Apple TV, but having to stream them while letting you download HDR and Dolby Vision, and even HD streams, just doesn't sound right. So if your internet connection isn't up to scratch at the time, or there's congestion at certain times of the day - oh well, you won't be able to watch 4K movies because you can't download them before hand.
In Apple's own support document, they state: "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". You'll also need a 25Mbps connection in order to stream 4K in the first place.
Apple's new 4K-capable Apple TV comes in just 32GB and 64GB versions, so you wouldn't be downloading 4K movies and storing them forever, but it would be nice to be able to download at least one 4K movie and store it locally so it doesn't sit there and buffer if you have a 25-40Mbps internet connection, or experience congestion at all.