I must agree with the melee attacks. 80% of my kills are melee. On the "gauntlet" runs in L4D2 maps you can make it with more health reserves when you melee through the crowd as opposed to using a rifle or shotgun.
Hordes:
If you are fighting a horde rush: run to a room so they can be funneled through a doorway or a hall, two players in front squatting with melee weapons and two in the back firing over there heads will decimate a horde. If you are stuck out in the open: back against a wall or corner with the same set up.
Tanks:
The best way to take them down is setting them on fire. The tank will usually chase one person at a time so: spread out and the other three chase the tank firing constantly. If the tank switches to chasing another person, just adjust to the three that are not being chased and keep firing. Firing from all sides seems to be the best.
Heros:
The definition of a hero is: "The player that gets everyone else killed". Saving a hero from attack while he/she is running out front trying to prove how great they are will get half of the team killed and leave two alone to fight. On the harder difficulties: this will make the game impossible to win. YOU HAVE TO STICK TOGETHER !!! That is the only way to keep your teammates standing. The group motto is "No One Left Behind" (N1LB). If a player constantly runs out front and makes the game miserable, they wont be asked back to play again.
Friendly Fire:
This is a problem that is hard to resolve. When you are being attacked by multiple entities, it is hard not to hit the other players. Melee in close quarters seems to keep the FFs down quite a bit. Try not to run across the other players field of fire. This is usually a problem when the players have not played together much. We should start talking about this as we progress through getting familiar with players habits in-game. If a player is firing, don't try to jump over the fire or run in front of them. Go behind them or wait till they stop.
I usually squat down to fire for accuracy ... this is a sign that I am going to fire. I have a tendency to stop at corners and scan for entities before heading into a room, hallway or open field. I have had players walk in front of me just as I pull the trigger. I am not a fan of running into an area before I scan it. Who knows what is in there. Close doors behind you! It will give you a few seconds to kill the entities before they enter the room and keeps them from catching you from behind ... especially in versus.