Although slow on the uptake, Google had earlier incorporated non-HDR 4K content into Google Play Movies library and now looks to be expanding that to HDR as well for a few select Movies and TV shows. This update is currently restricted to only US and Canada, with a global roll out in the near future.
The new update will work with HDR-capable phones such as the Sony Xperia XZ Premium, LG G6 and Samsung Galaxy S8, but Google has also focused on TV monitors with HDR support via its Chromecast Ultra. As long as the internet speed is strong enough (atleast 15 mbps according to Google), Chromecast Ultra owners can stream 4K UltraHD titles on their HDR supported TV monitors. If you happen to have internet that’s a bit slower, your movie might play in HD or SD quality.
There haven’t been a tremendous amount of 4K releases on Google Play so far, but the majority of those 4K movies are also now available in HDR. Google will automatically play content in HDR if the connected streaming devices and TV have the ability to do so. But if you wish to see what's in HDR right now you can try simply searching for "HDR" in the Play Store. From there, Play will automatically pull up a curated list of HDR-enabled movies.