what should I buy?

What kind of games do your group like?

http://store.steampowered.com/app/469820/Genital_Jousting/

Maybe Terraria?

broforce
borderlands 1-2-pre
dead island 1-2
helldivers
starbound
torchlight 2
trine 1 (only local), 2-3 (online)
vermintide
zombie army trilogy

- over $13
killing floor 2
dying light

Duck Game, Lethal League, Ultimate Chicken Horse, Birdsketball, Battleblock Theater, Castle Crashers, Frog Climbers, any of the Jackbox games, Party Saboteurs, ROCKETSROCKETSROCKETS, and Speedrunners are all good options, all of which support 4 players

...local co op anyway. Did you mean local or online?

if you can find a copy of Ultimate chicken horse somewhere thats fun for a few hours.
in a couch situation any (pointing to #3 tho) of the jackbox party pack games are fun (although I recommend playing them slightly drunk for the best experience)