It seems that some movies from various Disney-owned franchises are available in 4K via iTunes. This includes things like content from Marvel and Star Wars. MacRumors reported that the option is currently only available in the U.S. and Canada.
4K Disney Content From iTunes in U.S. And Canada
Indeed, from here in the UK I had a look at various Marvel, Star Wars, and Disney movies. They all showed HD availability, but not 4K, in the TV app. Content from other producers, including Apple TV+, was clearly marked 4K. You can, of course, get Disney content in 4K via Disney+ (and Vudu), so perhaps limiting its availability on other platforms like iTunes/Apple TV is a tactic to encourage further sign-ups to the streaming service.