So the 4K situation is a mess. This is what I've been able to gather so far:
Movies Anywhere: Doesn't support 4K at all. If you try to watch a movie directly from the MA app or website, you will only see 1080p.
Apple: Supports 4K, HDR10 & Dolby Vision. Does not separate HD & 4K purchases. If you buy a movie outright on iTunes & there's a 4K version available, you get the full 2160p copy of the movie. If you transfer an HD purchase from another retailer through MA & the movie shows as 4K on iTunes, you only get a 1200p copy, with the ability to upgrade to full 4K by using the "send as gift" option, sending to yourself & redeeming the code that is sent to you.
Vudu: Supports 4K & Dolby Vision. Separates HD & 4K purchases. Buying a movie in 4K on Vudu should get you the 4K version on Google Play & MIGHT get you 1200p in iTunes (awaiting confirmation on this). Unsure about Amazon. If you buy the HD version of a movie, you can upgrade to 4K for a fee, but that upgrade won't transfer to other retailers. Vudu also has a disc-to-digital program where you can convert your DVD's & Blu-ray's into digital copies for a small fee.
Google Play: Supports 4K & HDR10. Separates HD & 4K purchases. Buying a movie in 4K on Google Play should get you the 4K version on Vudu & MIGHT get you 1200p in iTunes (awaiting confirmation on this). Unsure about Amazon. Cannot upgrade HD purchases to 4K.
Amazon: Supports 4K & HDR. Separates HD & 4K purchases. I'm going to honest, I have no idea what's going on with Amazon as I haven't seen enough talk about what happens with it's 4K content. Will update when I get additional information.
Please correct me if I'm wrong on the any of the information.