Nice, okay, I see the issue. In the game, it says the controls, but the controls for players 3 and 4 are swapped-- it was telling me to use T and G, when I actually needed to use numpad 0 and 1. You might want to fix that.
Otherwise, cool game! There are a lot of overlapping systems at work-- I love that each player can really customize their character, between the stat system and all the different weapon options you've provided. Combine that with this being a cooperative multiplayer game, it really opens up the opportunity for different players to sort of 'specialize' in different things and work as a team! The game is too short and doesn't have enough enemy types to really capitalize on that, but I assume that's because of time restraints. Also, the instructions aren't as clear as they could be-- you could make the best game in the world, and if nobody understands how to play, it won't matter. That's why it's important to have people other than yourself test it out-- it can sometimes be tough, because of course you understand your own game. There are little things that you could also fix, like the grenades take too long to explode as it is right now. But I think you have the skeleton of a really good game on your hands here, and it would turn out great if you built on it a little more!