If you search for 4k or 4k HDR videos you can choose 2160p and 2160p60 HDR options. In order to choose the video quality, you need to tap on the video itself and open a 3 dots menu. After this, you need to select a Quality setting option and choose 2160p from the list.
This feature is available to all users. At the same time, you can become a beta tester for YouTube for Android as well.