The folks over at 9to5google have posted a quick how to guide on how you can force 4K when playing Stadia on the Chrome Browser.

Using a mix of information from the official Google Stadia Help Pages here and here - they were able to compile this simple list of steps to first check if your system is able to support 4K and also how to turn it on if so:

In your Google Chrome address bar, type chrome://gpu and press Enter.
Scroll down to Video Acceleration Information.
If you see Decode VP9, your computer supports VP9 hardware decoding.

Don't forget that you'll also need a 4K monitor, a 35Mbps or more internet connection and to have set the Best visual quality data usage option in the Stadia stream settings via the app. You can confirm the quality by opening the in-game menu (Shift + Tab) and clicking Connection where a green 4K icon should appear.

Source: https://9to5google.com/2020/03/13/stadia-4k-web-re...