iTunes movies will have a 4K symbol if they have it (and you have the rights) in 4K.
The thing is with iTunes/Apple - they upgrade pretty much anything you have rights to, to 4K if they have it. I have Blu-rays I've turned codes in for, that got a free upgrade to 4K in iTunes. When I started turning in digital codes somewhere around a year or so ago and realized that, it pushed me toward accepting that streaming is going to be the future and physical disks are going away. That was inevitable anyway but the quality upgrade completely changed my mindset. Before I was like Craig and others who wouldn't move away from physical media. That was kind of an epiphany. If a big company like Apple is going to not require me to double dip (or triple or whatever) to get that upgrade, it's a big win. I think streaming will at some point surpass physical in quality.
Obviously there may be some question about the provenance of some of the 4K content in Apple's library for movies that don't have a UHD release. I haven't researched what that means - is there some kind of cheap up-res being done just to have a "4K" version? Maybe. But at some point if that movie gets a proper 4K rescan/remaster done I bet Apple has it.