The 4k support feature was already available on iOS.
Finally, all Android users can now play videos onYouTube in 4K quality.
However, now the app has rolled out the option of watching any content in 2160p or 4k.

However, the 4k support feature is already available on iOS.
As per the latest reports, Google is adding a new streaming option on Android.
How to Play 4K Videos on YouTube on Android?

If you want to play any video in 4k on YouTube then you must have a stable internet connection.
As 4K is the highest resolution offered by YouTube and it needs fast internet.
you might check on your phone, whether you have received this feature or not.

All the Google Pixel devices including Pixel 4a and Pixel 5 can stream 4k videos.
A Reddit user posted a screenshot of the Youtube app having an option of 2160p.
One of the users said,
YouTube is sending you the amount of data needed to stream 4k.

Though its probably very hard to tell the difference on a small phone screen.
The new feature is a server-side update and it works on 15.05.35 version of the YouTube app.