TANK

The tank's job in left 4 dead is to incapacitate as many of the weak-bodied survivors as his steroid shrunken head can manage to locate. The beginning movie of Left 4 Dead actually touts a perfect saying for what you should always be doing when you see a tank.

Louis: RUN OR SHOOT!?.........RUN OR SHOOOT!?
Bill: BOTH.

If you are EVER fleeing in left 4 dead, swing your gun around like Rambo and spray in the direction of what you're fleeing from.

ON THAT NOTE.
When a tank arrives, be absolutely sure to stay away from cars and forklifts. The tank will punch these and launch them into your group, instantly incapacitating everyone who gets touched by them. Also, if at all possible, light the tank on fire. There are several ways to do this, the well known two being Molotovs and gas cans. However, if there is a flaming hobo barrel around, you can use it as a source of fire as well. It requires a bit of sacrifice as someone must stand right behind the barrel as the tank approaches. He will either punch you or climb onto the barrel, then punch you. Either way, he should light up and be dead within 60 seconds.

If you encounter a tank in a small area, be sure to melee him to get him to spread the damage around. Letting one person take the damage may be a good idea, but in advanced and expert they're basically guaranteed to die. Trying expert with only 3 survivors is a little like trying to disarm a nuclear weapon with your teeth.

Not a good idea.

Auto-shotties are the best way to take apart a tank. If everyone in the group has one you can blow through a tank in as little as 5 seconds. This does, however, take a hefty amount of cooperation.
As always, your group dynamic and "stickiness" will determine whether or not you manage a terribly placed tank fight.
Happy hunting!

Hunter

Are you the hunter or the hunted? This question may seem ridiculous in a ZOMBOCALYPSE (zombie apocalypse) but, when dealing with hunters, it is the ONLY question. You must learn to treat hunters like you would treat Pumas or Kodiak bears when faced with them in the wild. Those animals are hardwired to chase something that runs, and promptly tear it apart. Hunters are hardwired to chase survivors and promptly tear them apart, all the time, but can be defeated using the same techniques as you would against a Puma, Kodiak, or other large animal.

First, Listen for where the hunter may be. They run silently but as soon as they start crawlin around all creepy-like, they begin making kitty cat noises. Being aware of your predator allows you to prep for the confrontation. You don't need to pop pills or heal up, necessarily, but being aware of the location of the kitty will largely help your efforts to not die.

Second, use corners to make your target lose sight and HAVE to get close. Hunters get a large initial damage bonus if they land on you from a long leap, so shortening the gap may just save your life. Past that, hunters are great punching bags. Their soft hoodies allow you to maliciously drive the but of your rifle into them without breaking your nails. Using corners and walls, the hunter will eventually get close enough that you can punch his face without being injured. Also, if you've got one of the better weapons, wait for him to come around the corner and then treat him like you would an attacking puma.

Shoot him in the face.

Third, if you're out in the open, away from your friends, and you see a hunter, do NOT turn and run. Maintain eyes on the target. Open up by firing at the hunter, regardless of the distance, as being hit will cause them to leap away and come back for a retry (hopefully still far away). When you hear the scream and see the beast flying through the air, you have two options. First, sidestep. This only works if the hunter is rather far away because they have remarkable precision and speed. Just strafe out of the line of fire a few feet, then return to punch the Hunter's face and shoot him in the neck. Second, move TOWARD the leap. Hunters rarely go over your head, and that is not at all the intent here. Humans surviving animal attacks are often let go because they put up too much of a fight, not because they attempt to dodge and then run. Hunters function the same way. When he's flying toward you, walk forward, and when you can estimate he's about a second out from pummeling you, punch. I find that about 1/5 times this works for me and gets me a solid Jump Stop. It may not be a super high percentage, but learning how to work it even 20% of the time saves you and your teammates from death 20% of the time.

Hunt the hunters, then punch their necks and shoot their faces.
Just like pumas.

Smoker

Smokers are mild aggravations, at best. Their damage doesn't work from them acting solo, as a hunter, tank, or boomer's does. BUT. That is not to say that they're not formidable opponents. Smokers shoot their nasty tongues out and drag survivors away from the group. Occasionally, and unluckily, smokers will drag a friend off a ledge or through a fire or into a boomer's vomit. These are the sticky situations you ought to avoid. When you hear a smoker coughing, open fire where you see the green plumes of smoke or where you last heard the noise. They gimp along with a constantly tilted head, probably from the tobacco, and can be found on roofs, behind cars, or in forests. Smokers always spawn BEHIND the survivors. Hunters, boomers, and tanks, however, spawn in front of your merry band of travelers.

To deal with a smoker who has grabbed you, immediately spin so you can see the tongue and SHOOT IT! Smoker's tongues can be shot off of your friends by aiming at the tongue leading from your friend's torso to the smoker's mouth. In my experience, you can't shoot the tongue that is ON your friend, but can only punch it off. If you're far away from a buddy who is getting smokered, be sure to open fire on the pink line you can see leading him to the maw of the smoker.

On that note, whenever you're leaving an area where no one can return, make sure you all leave together. This is especially important in versus situations. While three people have jumped down, one may be lackadaisically checking out bricks on the walls and end up being smokered to a location no one can see or reach. This is, as you may suppose, bad news bears.

Shoot first, ask questions later. You are the only survivors.
Except for the dude in the church, but hey, he's a bastard.

Boomers

Chances are you haven't been vomited on by a morbidly obese person, or exploded them and gotten coated in their entrails. Left 4 Dead offers plenty of opportunities for you to do just that.

In case it was something you've been dying to do.

Boomers are rough. They are by far the most strategic of the zombie specials. Boomers come flying off roofs, tumbling through doorways, or jumping out from corners to vom all over you and your homies. When they throw up on you, or you shoot them to close, you get covered in boomer bile. Boomer bile, for whatever reason, is irresistibly tasty to zombies. Zombies will travel for miles to get to someone covered in boomer bile. Zombies run like Kenyans. Once they find that person, all they really want to do is lick it off, but they often work themselves into such a frenzy thinking about it that all they can do is scream and try to scrape it off your flesh.

Needless to say, this is not so fun.

If one person in your group gets vommed on, it is everyone's responsibility to guard that person from the flood of zombies that will try to lick her to death. Zombies have rough tongues, like cats.

When a person is covered in bile, shove them into a corner and have them melee like there is no tomorrow. Which, considering the situation, may not be far from the truth. Station one or two other survivors next to the person to help melee and blow the brains out of zombies, and place another survivor a bit away to offer some non-melee gun fun. Closets are also great cover for boomer situations. Back into a closet, two in back, two in front (crouched). The front peeps melee constantly while the back spray bullets forward. This works not only for boomers but also for epic events like exploding a wall or accidentally setting off a car alarm. It works in all situations, except those involving tanks.
If you hear a boomer, feel free to fire in the direction of the flatulence and belches. Boomers are delicate flowers, and as such are easily punctured. Bullets go through walls, doors, windows, and other zombies. Hunting rifles go through just about everything. Including delicate flowers. Spray bullets and you'll save your team a zombie rush.