Google has started rolling out 4K content in Google Play Movies, as was promised back when the company announced the Chromecast Ultra. For select movies, you will be able to purchase or rent them in 4K resolution. The prices are higher than the SD version but not significantly so. 4K content so far is seemingly limited to Chromecast Ultra and Android TVs with 4K support. If you're on a smartphone or web browser then you will be limited to HD resolution. The 4K option is rolling out slowly across regions and accounts. Source
Source: http://www.gsmarena.com/4k_content_rolls_out_in_google_play_movies-news-21730.php
