Tried an experiment last night.
I have a Philips 4K Android TV and a Mii Box (4K version). The Prime Video app for some reason cannot be installed on either of these from the Play store, so it has to be sideloaded (not an uncommon problem).
I have the same version sideloaded on both (current apk from the nVidia Shield)
On the Mii Box, when I go to the TV section, I've scrolled all the way to the bottom and there is no Prime 4K section. But, I can watch any of the 4K shows that are already on my watchlist, in 4K/HDR I should add
On the TV however, I do have the 4K section!
I was able to add Homecoming from here to my watchlist and the 4K version shows up on the watchlist on the Mii Box