The answer to your headline question is no.
A 1080p DTS-MA movie Blu-Ray when ripped runs about 20-30 GB. An uncompressed 4K movie runs about 100 GB. You have space to download maybe 2 non-compressed movies on a 64 GB Apple TV and there is no space for even 1 4K movie.
If you use the heavily compressed (audio greatly compromised) 1080p or 4K streams used by Netflix (and I assume iTunes) the sizes are better. Something like 4.7 GB an hour, so lets just call it 10 GB for a full 2 hour movie. 4K can be 18.8 GB/hour, 37.6 GB for a 2 hour movie, although h.265 may cut than down to 9 GB an hour, so let's call it 20 GB. So you're talking no more than 6 1080p or 3 4K compressed movies.
And if your internet speed is so slow, how long is it going to take you to download such large files? 60 GB at 2 Mbps is going to take something like 71 hours. And do you have a data cap? I backup my media server to the cloud so I burn between 2-3 TB of data a month.
^That sounds about right, I was going to suggest 18-30 GB compressed per movie.
I started to write something yesterday, lost it...the storage space required is surely a large consideration, download one 4K movie and your 32 GB ATV 4K has 12 GB or less to work with for apps, streaming and everything else.
When you're on a computer with a xTB drive, 20 GB doesn't seem like a big deal, but, it really adds up quickly, so, will we one day be able to download 4K to iTunes on a computer? Probably not.
The other consideration...by making the movie only available via streaming puts a bit of a limitation on it that the studios may have wanted as well, (to help prevent piracy).
I get about 15mbps. I would download the movie when im not home or when sleeping. I download ps4 updates and games no problem doing this.
I understand the concept, it's frustrating, I've lived in places with limited ISPs, max was 10Mbps DSL for a long time.
Apple recommends 25Mbps, but, I think it might actually work at a minimum of 15 (minus HDR, I think you need about 18+ for HDR), if you actually get 15 Mbps, you might be able to do it, if you do nothing else on the internet while watching the 4K content. (I think Netflix 4K content runs at about 15 Mbps from what I've read). You could try it, just keep your receipt and be sure you know the return policy from wherever you get it.
One thing to keep in mind, you probably won't be streaming all 4K content, there are limited titles at this point, much of what you'll watch is going to be 1080p anyway.
