I've only played this solo, but this game looks really cool and I think having this with 4 players (or even more??) would be an absolute blast, so I strongly encourage you to consider that. (There's a page on the wiki with details on four player adapters.) The concept is really neat and I hope you flesh it out with sound and perhaps more maps, ways to get more ammo, or whatever other ideas you can think of. The core here is really good.