Google Play is upgrading available movie titles to 4k resolution with HDR at no charge. The free upgrade follows Apple’s decision to do the same for iTunes customers when the Apple TV 4k was released. You should start seeing any HD titles you own on Google Play available in 4k on supporting devices.
As well upgrading video resolution, Google is making the higher quality available on beyond Chromecast Ultra and 4k Android TVs, with expansion to the Google Play Movies & TV app on supporting Samsung and Sony Bravia TVs Smart TVs and LG in the near future.
Google is part of the Movies Anywhere platform which shares most titles among other service providers including Amazon Prime Video, Apple iTunes, FandangoNow and Vudu.