YouTube for Android has added playback support for any content in 4K resolution. Until now, the YouTube app for Android limited the playback quality of videos to the device's native resolution, so only phones with a 4K display could play videos in 4K. However, from now on, this restriction disappears and everyone can choose the quality of the resolution.
Now, if your smartphone's screen resolution is 720p or 1080p, which are the most common resolutions, you can increase the video quality up to 4K, as long as the video provides that resolution.
The change has come in the YouTube app for Android. But there are objective reasons why this is an improvement that the user can benefit from. If your phone has a 1080p display and you play videos in that resolution, that doesn't mean the quality you get is optimal. YouTube, like all other online video platforms, applies compression to audio and video formats, as well as bit rate decrease to reduce the size of transmitted files. And the same goes for 4K videos.
This is the reason why a Blu-ray movie can take up tens of GB, while the same movie on Netflix, for example, takes up less than half. The limiting factor, of course, is storage, but also internet connection.
So, the change introduced by the YouTube app for Android allows you to improve the quality of the video even if you don't have a device with the appropriate resolution. Ergo, if your mobile screen is 1080p, you will see a 4K video with higher quality than 1080p, even if it does not reach that resolution. Yes, the same also happens on the PC.