I think it will be a long time before 4K is offered through iTunes. A typical 4K movie will run about 100GB in size (vs. 4GB-8GB in size for 1080p).
Apple will need considerable upgrades to its own infrastructure to support mass streaming and downloading of 100GB files. And internet providers won't be happy about all that extra traffic on their networks. 4K may have to wait until the next generation of home internet, IMO.
4K h.265 files will be approx. 2x the size of 1080p h.264 (as h.265 is around 2x as efficient) and HFR 4k files around 4x the size of 1080p h.264
So if a 1080p h.264 file (Current iTunes offering) is 8GB
4K version will be about 17GB
4K HFR version will be around 35GB (depending on frame rate)