Valve are now offering a chance to test a beta of the next version of Proton in their Steam Play system.
As a reminder: Steam Play is the system built into the Linux version of the Steam client to allow you to play Windows games on Linux. Compatibility varies from game to game, you can see the current titles that Valve have whitelisted in their original announcement.
You can track the changelog here. As for what Proton 3.7-4 has: support for Python 3, DXVK updated to v0.70, better keyboard focus for those on GNOME Shell, controllers hopefully no longer causing long delays on startup and more. Seems like a pretty good update and it's going to be fun to watch it progress.
Actually getting it show up is a little finicky. It didn't seem to appear until after I started a Steam Play title and then restarting Steam, so hopefully they will improve that over time too.