Even if you don't have a 4K display on your Android smartphone, you now have the option to consume YouTube content in 4K on Android. Reports of the same started coming in from February 15 where people with a 1080p or even 720p display had the option to play videos in 4K on the YouTube app. This feature has been available on iOS for some time now.
So what’s the advantage of consuming 4K YouTube content on a non 4K display? Well, to begin with, you will notice a sharper image especially if you have a smartphone with a nice FHD+ or QHD+ display. When the content plays back in 4K on a display with a lower resolution, the image is supersampled for the display making the image look crisper and sharper when compared to content played back in the native resolution of the display.
This won't be the case for everyone though. Playing back 4K content does take a little bit of horsepower under the hood and chances are, if you have a budget smartphone, your phone may stutter or lag when playing content in 4K on YouTube. Nevertheless, 1440p content can also look extremely crisp on a good 720p display. If the content on youtube is available in HDR, you will be able to playback the content in HDR on your display as well. How good HDR content will look on a non-HDR display remains to be seen.