A few weeks ago some people started seeing 4K (UHD) content in Google Play Movies, but this wasn't available for everyone and Google didn't list separate prices for the high-res videos. Apparently it was a test rollout of something like that, since the addition of 4K content to Play Movies has only become official today.
As usual with Google services that have to do with content of one form or another, this too is severely limited at the moment, geographically speaking. In fact, right now 4K movies are only out in the US and Canada. Oh, and only 125 movies can currently be watched in 4K.
You'll need to pay $7.99 to rent a 4K movie, and $29.99 to buy it. That said, not all movies can be both rented and bought - sometimes you only get one or the other. The recently launched Chromecast Ultra should make it easy to play back this content on your 4K TV if it isn't running Android. If you buy a Chromecast Ultra, you'll even get one 4K movie for free. Google is also releasing a Play Movies app for 2016 Samsung TVs, so now you have even more ways to watch your favorite movies and TV shows.