If you have a supported device
, I think your best bet is to sign up for Vudu, sign up for Movies Anywhere, and link Vudu and iTunes in Movies Anywhere. Buy the Disney/Pixar/Marvel/Lucasfilm content on Vudu in 4K HDR (UHD) for $25, watch it in 4K HDR on your supported device, and then if/when it's available in iTunes in 4K HDR hopefully you'll get it for free.
I've heard that if you buy a "HDX" or "HD" movie on non-iTunes digital services, you don't get the 4K HDR iTunes version for free even when it's available on iTunes. Not sure if that's true for standard Blu-Ray codes too. If you're using a code from a UHD Blu-Ray box, you do get the iTunes 4K HDR version when available.
I'm strictly digital since DVDs, and tried to stick with Netflix for streaming and iTunes for purchase as my only sources. This mess with Disney has forced me to branch out (and pay more)...
Hope that helps!