For many people, mobile devices have replaced desktop and laptop computers as the primary avenue of content consumption. The official YouTube app for Android is used by hundreds of millions around the world to keep up with their favorite channels. Video quality on YouTube has dramatically increased over the years with the introduction of 60FPS options and 4K resolution, though for the vast majority of people without a 4K monitor, YouTube 4K videos are simply inaccessible to them.

Some phones, such as the Sony Xperia XZ Premium, offer 4K resolution displays though this is by far the exception rather than the rule. Users of the Sony Xperia XZ Premium are able to watch YouTube videos in full 4K resolution, but most other devices cannot. If you have the connection to handle streaming a 4K video, why can't you choose to watch it? Fortunately with a simple build.prop modification any rooted phone can also watch 4K YouTube videos!

The above screenshots were taken on a Google Pixel in the official YouTube app. The Google Pixel has a 1080p screen, so by default the maximum video quality available in YouTube is 1080p. The video that we tested this modification on is MKBHD's "The State of 4K: 2017" video.

We confirmed that the higher quality video stream was being served by comparing the network traffic at 1080p versus 2160p. The 1080p stream resulted in 1.5 Mbps versus 4.5 Mbps for the 2160p stream. Furthermore, my tester subjectively claims that while the sharpness did not change, the color quality and artifacts substantially improved.

There is much conflicting information about the resulting quality of watching 4K videos on a lower resolution screen. Some sources claim it's better than watching native 1080p content, while other sources claim that downsampling will not change the quality. Try the modification out and see for yourself.

YouTube 4K Modification

The following requirements must be met in order for the YouTube app to display 4K content:

All you have to do is modify the build.prop file in /system using a root file browser (or by using an app like BuildProp Editor) and add the following line: 

        sys.display-size=3840x2160

Force close the YouTube app and clear its data, then reboot your phone. When you open up YouTube again, the app will think your phone has a 4K screen so it will offer these higher quality videos. If your phone doesn't have hardware VP9 decoder support, then you won't see a 4K video option, but at the very least you will see a 2K resolution option if you aren't already seeing that option.

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Many thanks to XDA Recognized Developer Rizal Lovins who originally posted about this method on our forums!