Everyone would prefer if they could always watch videos in the highest possible resolution, but technical limitations often stop us — a slow connection, a low-res display, DRM, paywalled subscriptions, or even a restriction put in place by the app to stop a certain platform's users from streaming in high res. For instance, Google used to limit YouTube videos to 1440p on Android, even if they were uploaded in 4K or more. That's no longer the case now.
YouTube users on Android can finally benefit from the same high-res limit as their iOS friends, and watch videos at up to 2160p (4K) HDR. As long as the uploaded video was originally 4K or larger, and had HDR, you get both options in the quality selection pop-up in the app. This seems to be true regardless of your phone's display, so you could waste your bandwidth streaming a 2160p on a 720p display and no one will stop you or tell you it's pointless.
Left: Video available in 2160pHDR. Right: Video available in 2160p.
The first sign of the feature going live dates back a few days ago, to February 15, then reports started coming in thick and fast after that. Now, all of our devices have the option, regardless of the account, country, or app version, so it seems to have rolled out widely. In case you don't see it, try grabbing the latest YouTube version from APK Mirror to make sure you're not behind.