There is a quirk with iTunes 4k that I’ve noticed in my experience. I have to dig up the link later, but iTunes 4k needs to be either: a 4k purchase (UHD disc or quality on, say, Vudu) that gets matched on iTunes, OR, an iTunes HD redemption that gets upgraded to 4k.
If you buy a regular Blu-ray but redeem on MA or Vudu, the movie itself will be playable on iTunes, but be considered a secondary purchase and NOT quality for a free upgrade to 4k. To ensure an iTunes 4k upgrade, you should redeem the code on iTunes DIRECTLY to get the iTunes bump, and then iTunes talks to MA to be useable on other devices/services.
The extra weirdness is that in iTunes, you’re told the maximum quality AVAILABLE, but you don’t get confirmation of what quality on iTunes until you play the movie on a compatible device.
My use cases that illustrate these quirks:
I have a UHD code of Wonder Woman. I redeemed on VUDU, and because it’s also available in 4k on iTunes, it matches. The iTunes listing says 4k Dolby Vision, and when I play it on my Apple TV 4k on my OLED, I get 4k Dolby Vision.
I have a regular Blu-ray copy of War for the planet of the apes. I redeem on Vudu. It’s HD on Vudu, and gets added to my iTunes library. Because I redeemed it on Vudu, it’s not considered an iTunes purchase, and I did not get a free 4k upgrade. The extra annoyance: when I look at the movie in iTunes, it tells me 4k HDR is available, but when I play the movie itself, it plays only on HD. So what I should have done is redeemed it directly on iTunes first to get a free iTunes 4k upgrade.
Net/net: if you’re buying a UHD movie, it should be 4k everywhere. If you’re buying a Blu-ray, redeem the code on iTunes to get the free 4k upgrade, and iTunes will also add it on HD to your other MA services.
If anyone bought a normal Blu-ray, redeemed the code on either Vudu or MA, and got the 4k bump in iTunes during playback, I’d love to hear it!
Note: Reddit link (lol) with supposed info from Apple that maps to my experience above: https://www.reddit.com/r/appletv/comments/77gax4