Google has added support for 4K HDR streaming for Play Movies. However, the support is currently available for users of the Chromecast Ultra streaming dongle. HDR (High Dynamic Range) video essentially improves color reproduction and contrast in videos. Compared to HD quality (1920 x 1080 pixels), 4K UHD has a resolution of 3840 x 2160 pixels.
Uers will need a compatible HDR television and a Chromecast Ultra dongle to stream 4K HDR video on Google Play Movie. It must be noted that HDR will not be enabled on SD or HD videos. The HDR rollout is being enabled via support from two major studio partners, Sony and Warner Bros. Currently you can find HDR movies like Kong: Skull Island, The Lego Movie (and The Lego Batman Movie), Mad Max: Fury Road, Suicide Squad, Fantastic Beasts and Where to Find Them
Google says HDR will first be available in the U.S. and Canada on Google Play Movies & TV. HDR playback is currently not available on Android TV.