We're looking to play some Sniper: Zombie Army Triology.
It's hand-in-hand with L4D. Even supports it with some crossover content.
Four player cooperative sniping nazi zombie brains.
A gamer's wet dream.
The game is expensive as balls on Steam's store ($40)
But you can pick up a copy of the game on G2A . com
It's a site that hosts people gifting games out of their inventory for cheap (seems on average about 75% off).
I was able to get a copy of the game for about $8.
https://www.g2a.com/zombie-army-trilogy-steam-cd-key-global.html
If you're down with playing some of this Sephie and Grey are going at it and we could use some jolly cooperation!