I think 4K files should be at least 10 GB for a 2 hour movie. I do not have any technical reasons for that choice! It is based simply on finding smaller 4K encodes and not seeing any improvement over a good 1080p file. However, I must admit that I personally have pulled back on downloading many large files because of lack of storage space. So take my opinion with a grain (or a big hunk) of salt! I am not really the right audience.
Skalman is absolutely right that H265 is ideal for 4K. Movies on 4K disks are already coded in H265/HEVC anyway. Using H264 would be going backwards in quality and compression! (I am only familiar with AV1 from some YouTube videos.)
I think that if someone can not play H265 codec then they also may not be equipped for 4K playback in general? I mean, they could play a H264 file that was "4K" but their monitor/TV would not actually show 4K quality, just 1080p. So they might as well get a 1080p version instead. If they want the highest quality image, then they should upgrade the equipment too.
What I don't understand is the 480p files I find that are encoded in H265 to try to make them as small as possible! As far as I am concerned, 4K demands H265 and SD demands H264 (or older!). Flip a coin for 1080p.
Maybe some day we will get Internet speeds in TB/s, hard drives in the PB range... and files that are so efficiently compressed that a 2-hour 4K UHD movie with Atmos audio would only be about 700 KB.

But not yet.