...What's four? For our current project the decision of how many players works best in a cooperative game mode is still being decided.
A two player co-op game tends to create a personal relationship between the two players. Where four players gives more of a team based or party feel to the game, At least in my mind.
Obviously being able to have the option to play 2 - 4 players would be the way to go as it lessens any restriction of how many friends you can play with, but would the game need to change depending on the number of players participating?