If you don't want to install an extension to fore 4K VP9 support on Stadia on desktop you can copy and paste the following code into the Chrome Developer Console (ctrl+shift+j) before starting a game. This will tell your browser to use the VP9 codec as well as tricking your browser to load the 4K video stream for the game you are playing.
// set local codec to VP9
localStorage.setItem('video_codec_implementation_by_codec_key', '{"vp9":"ExternalDecoder"}');
// set 4k dimensions and screen values
let x = 3840;
let y = 2160;
let screen = [
['availHeight', y],
['availWidth', x],
['height', y],
['width', x]
];
// force new screen resolution
screen.forEach(([prop, value]) => {
Object.defineProperty(window.screen, prop, {
value,
configurable: true
});
});