YouTube seems to have quietly rolled out the option to watch 4K/60p videos on any mobile device even if it doesn’t have an Ultra HD display. Though you won’t get quite the same visual quality you’d get with that higher-resolution screen, those who have lesser display resolutions will still benefit from turning on the 4K video option.
The change was recently discussed on the /r/YouTube subreddit, where users noted that they now have the option of selecting 4K video playback even if their device lacks a 4K display. Assuming you have Full HD+ phone, boosting the video resolution to its max should still provide a better visual experience compared to watching a 1080p video — though, of course, the best experience comes with having a 4K device.
At this point in time, it’s unclear whether the ability to play 4K videos on devices that lack an Ultra HD display is an intentional change or one that may disappear in the near future. Google hasn’t commented on the change, though many users seem happy and it wouldn’t likely go over well if it were suddenly taken away again.
The feature appears to be available specifically on the Android app; it’s unclear whether all low-resolution devices have access to the 4K video playback or if it is still limited to Full HD / Full HD+ screens. What’s the point of playing 4K videos on a non-4K device?
Users claim that despite not having a 4K display, playing Ultra HD content ultimately provides better fidelity over 1080p videos. This benefit is likely a result of lesser compression artifacts, as well as the downscaling that has the upside of increasing sharpness. It seems those who have high dynamic range displays will also be able to watch the videos in HDR.