I think we should address the topic of formations and roles. People tend to bugger off, and frankly the partner system still leaves them vulnerable. All four people should be reasonably close at all times; I know that makes them vulnerable to Boomers, but an intelligent squad can overcome the occasional vomit (ie Don't Fucking Shoot Them).
I think the healthiest guy should always be in the lead, first of all, and if anybody in the squad is firing a gun, he should be crouched. The lead guy should probably have one of the ARs and his primary focus should be taking care of boomers. Nobody in the group should shoot a boomer except for the leader. He's the most likely to get vomitted on. Whether it happens or not, he has to beat the boomer back and take him out and everybody else should avoid him when that's happening. The zombies are all going to go straight for him; instead of hanging out near him, it's better to set up angles so that the zombies run right across your field of fire.
Behind him should be two guys, one with a sniper rifle and one with a shotgun. The shotgun guy should protect the group from hunters; usually, in a big fight, if someone gets huntered, everybody turns and tries to help. This is noble, but in a large fight, the zombie horde is the real threat and requires due attention if anybody's going to make it any further. Therefore, this is the guy people should call if anybody's been tackled. He should always be close enough to help. As for the Sniper guy, he should be watching for smokers and setting up witch kills (if necessary). I don't know much about the Hunting Rifle's capabilities so I will leave further pontification up to Noro and Zed.
The last guy should have a AR and needs to watch the rear like he's an ass inspector. The AI Director LOVES throwing shit behind everybody, and Versus opponents find great pleasure in sneaking up from an unexpected angle. The last guy should be expecting all of those angles. He should also be in charge of pipe bombs, molotovs, gas can/propane tank traps, etc.
As always, if you're under attack, crouch. And even if you're not, it's a decent idea. If you're safe, the guy ahead of you should crouch so that you have a good angle over him.
Of course, these are just general guidelines, and in the real game people aren't going to be super-organized (for instance, when the Tank spawns, this rulebook goes right out the window). But it's worthy food for thought.