When you update the app, you’ll find a new Video tab at the bottom of the screen. This new test will play a few short clips so the app can measure how good (or bad) the speed of your service provider is.
Once it’s finished the app will provide you interesting statistics, such as load times and buffering percentage. And yes, it will tell you the max resolution that’s best suited for your carrier’s mobile speed.
It’s a nifty feature to check if your service provider can certainly deliver the speeds they promise. As for actually watching 4K videos on small phone displays, however, it’s arguably excessive and impractical.
Here’s the Speedtest by Ookla download link to the Google Play app store.