Movie purchased on HD

I specifically remember that title being available in 4K upon release here, but there is a possibility that it differed for other locations.

When a movie is released and available in 4K then that will be the best version available. It will then only be downgraded in the following scenarios (non 4K display, inadequate network connection or iTunes Store video settings, when downloaded). For titles originally purchased in HD, once a studio upgrades it that same version will be available in your library.

Do you have other titles in your library that you can check to see if they are in 4K? If you are able to isolate the issue to this title then it's best to contact Apple support to initiate an investigation, but if you see this elsewhere then check your video settings - make sure you have quick start set to ON and best available selected for resolution (Apps > iTunes Movies and TV Shows). Also, check Speedtest to see what connection ATV is currently receiving.

Movies that are in HD will automatically be upgraded to 4K when available, it is up to the studio as to when/if that occurs.

What's the title of the movie?

So that title was already available in 4K when it was released so no upgrade required. Is there a reason why you think you only have it in HD? Can you provide a screenshot?

Ok then really the only thing you can do is to contact Apple support, to ensure they are investigating the issue with this title. Good luck.