As previously announced, Stadia requires a stable internet connection to stream your favorite games online. Google Stadia will be offering up to 4K resolution but in order to reach that resolution, players also need to meet the internet speed requirements.
Starting at 10 Mbps, which is the minimum internet speed, players will be able to stream and play a game on a 720p resolution. The 1080p resolution starts at 20 Mbps internet speed and also features HDR video at 60 fps. And for the highest resolution, 4K gaming stream, players need to have at least 35 Mbps stable connection, also offering HDR video, 60 fps, and 5.1 surrounds.
You can visit the official Stadia website to check if you qualified for the 4K resolution. Google also confirmed all the games that are coming to Stadia during its launch. Check this post for the confirmed Stadia games.