Multiplayer shoot em up games with 4-player support

The best multiplayer shoot em up games with 4-player support, including Mercenary Kings, Netherspace 2, Airstrife: Assault of the Aviators. See if the games support online, couch, and split-screen play, whether the game has a cooperative campaign or separate multiplayer modes. Find out the number of players supported and check out ratings of multiplayer games.