This is definitely an interesting change in the YouTube Android app, as it now lets some users playback videos at up to 4K resolution at 60FPS with HDR on devices with lower res-displays.
This feature is seemingly a server-side update as I’m using version 15.05.35 of the YouTube app and not the latest 15.06.35 build which released on February 13.
Last year the iOS YouTube also app gained 4K playback with the rollout of iOS 14.
In one user personal testing with a Galaxy S10e playback at 2160p worked well though when he switched to a Galaxy Tab S5e playback fared much worse with chopped frames resulting in more of a slideshow than video playback.
And It also remains to be seen which devices will gain this new feature though it’s a welcome addition.
Even on smaller FHD+ smartphone screens, the 2160p videos look noticeably sharper with less compression due to the higher bitrate.
So now we leave it all to you guys in the comment section to let us know if your smartphone or tablet gives you the option to playback 4K videos on YouTube.