STRAFTAT is a free to play arena shooter that's quite popular, and it just had a big content expansion with support for 4 players. It has a Native Linux version too.
Originally a 1v1 frag-fest, it now supports teams along with free for all up to 4 players total. This can be a mixture of 2v2, 1v2, 1v3 and so on. While there's a lot of online shooters, STRAFTAT sets itself apart from the back-to-basics arena gameplay with each round being a completely different environment with a different set of weapons. It's complete chaos in the best of ways.

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Highlights from the update:
- New Gamemodes : Teams, FFA (both : up to 4 players).
- New lobby settings (Friendly fire, mid match joining, player outline).
- Reworked and improved Main Menu interface.
- Lots of improvements on Logs in killfeed.
- Added 4 Player Spawn Points to each map.
- Weapon Randomizer Improvements (Customize weapon pool, spawn rate…).
- Native Map Playlists.
- Added Speedometer (Enable in Graphics Options).
- Map Playlists export/import (easy to share your playlists online).
- Killcam improvements.
A fair revenue model too for a free game. There's a DLC with more maps, weapons and cosmetics. The maps and weapons are enabled for any players as long as the host owns it. There's also a new supporter edition DLC with this update that adds more cosmetics.