Yeah, unfortunately you can't download the 4K versions. But for me that's an OK tradeoff since I've never once had to use the downloaded version to actually watch something on my AppleTV; everything has still been available in my account for streaming. Not to mention that over half of my library is movies from the 1930s to 1960s - way before 4K was even a concept. So except for a handful of recent movies, I've got the highest form downloaded anyway.

I have a Mac mini that exists almost exclusively to be the backup for all my digital content should I ever need it. All the movies are downloaded to a 14TB external drive which is then cloned to another 16TB external drive and also fully backed up to Backblaze.