Borderlands will last you a little while, and the DLC (this is gonna get pricey) extends the game quite a bit. (General Knoxx is the only DLC worth it IMO)
I played Borderlands a rediculous amount of time right after it came out, on PC. Played through it 3 times with one other person, then we got sick of it. I never played with 3 others, though, which I both imagined and heard of it to be total chaos (shouldn't be a problem if your party is organized and doesn't mind sharing).
As a big fan of Vegas, I also recommend Vegas 2 (more guns, maps, and customization). Especially since it's cheap.
L4D2 will get old quick, unless you and your buddies love mindless fun.
I enjoyed World at War -- The coop campaign is pretty fun, and the zombie maps require a bit of strategy to actually last. Also pretty cheap.
Maybe you should look into Lost Planet 2 when it comes out (May 11th I think), that should be good fun for 4 people.