The cover 4 defense is basically the same thing as a prevent defense and is hardly ever used. It's gotten such a bad name that coaches don't want to admit that they've used it ("We weren't in a prevent" ranks right up there with "I did not have sexual relations with that woman" in the book of lies.)
If you've followed along with the other cover defenses you should be able to recognize that a cover 4 means 4 defensive backs. This is sometimes called quarters, though not often (nickel, dime, quarter).
At it's most basic the cover 4 is a 3-1-7. That's three down linemen, one linebacker, and 7 DBs (almost always three safeties). It can take other forms too. Bill Belichick has used a 0-4-7 (no down linemen, four linebackers, seven defensive backs).
Contrary to popular opinion the object isn't to prevent the gaining of yards, or even a score (depending on the situation). Rather the object is to use up valuable time. Thus the prevent should only be used in situations when there's very little time left at the end of a half or the end of a game, and should only be used when the offense has to score a TD. There's one other time when a cover 4 can be useful but it's not common in the NFL.
Classic cover 4. The two inside CBs are generally zone, with the two outside CBs playing man and the SS floating over to help out.
Belichick's Cover 4
When running a cover 4 or "prevent" there are two extremely important things to keep in mind
First: Keep the action inside.
The CBs on the outside have to force the WR towards the inside of the field. It's vital that the clock continues to run. To quote Bill Parcells (a proponent of the Cover 4)
"The philosophy is to trade modest yardage for time, the main thing is you don't let the guy get out of bounds."
The reasons for this should be pretty obvious. If you've got a minute left, you don't want the WRs stepping out of bounds to stop the clock. You want the opposing team to be forced to use up their timeouts (if they have any), or be forced to use a down to spike the ball.
The other reason for keeping the action inside might not be so obvious, but it leads to our number 2 most important aspect of the Cover 4
Second: No big plays!
Everything has to be short, preferably 10 yards or less. A first down is ok (because the clock starts again as soon as the chains are set), but a play of 15 or 20 yards (or more) is bad news. If the CBs keep everything inside it helps limit the big plays because the WRs can't get past the defense. They'll be met by another CB, one of the safeties, or a LB.
How do you beat a Cover 4? The easiest way is to line up trips (three wide receivers) on one side. This forces one of the safeties to cheat over, leaving the outside CB manned up. An accurate QB can complete that pass for big damage.
Here's a diagram of a cover 4 where one LB is removed with the four defensive linemen left in the game. This is probably a situation you'd use in a normal game when the offense moves to a spread type formation. In a two-minute drill situation you'd go down to three or even two defensive linemen.
As you can see, here the two outside corners are on man coverage. The nickel back is lined up over the second WR. The safety has two options--he can help cover the outside guy, or cheat down to cover the inside guy. If he cheats down there are two WRs on man coverage and the middle safety has to come clear over to cover the second WR.
If he doesn't cheat inside the linebacker is left covering a WR and that's a potential mis-match.
Benefits of a Cover 4
Obviously the biggest one is preventing the big play. It's the "bend don't break" philosophy of defense. Another key benefit of the Cover 4 is limiting the damage that a spread offense can do to you. If you stayed in your base package when an opposing team rolls out a spread offense you'll get killed because most LBs can't cover a WR man, and if you're running a zone defense you'l be exploited in the seams.
However most NFL teams rarely run with a spread offense except for in a 2-minute drill type of situation.
49er Cover 4
This will bring back painful memories but it's instructive. In this play we're rushing four (three linemen and one LB), keeping one LB in the middle for zone (Willis), Running man coverage on the three WRs (Bly, Clements, and Spencer). Back deep are Lewis, Roman and Dashon (I'm one of the best safeties in the league) Goldson.
Lewis is covering the middle deep of the field, Goldson and Roman are responsible for the deep halves of the field.
This is really a cover 5 but the same principle applies except that we'd have one extra LB and one less safety.
That's Spencer coming across the field, Willis is out of it, Clements is the CB on the right, Lewis is the shallow safety and Goldson and Roman both have responsibility for the deep ball. Goldson had time to make a play on the ball but he gave up thinking that Roman had it covered.
The Cover 4 actually worked in this situation. The last play came from the 32 yard line, and Favre had to throw from the 40 yard line. Much as I hate to say it, nine out of ten QBs in the league probably don't make that throw. Of course nine out of ten safeties will probably draw a PI call on themselves to make sure that there's no score. (Rather the ball placed at the 1 yard line with 5 seconds to go than a TD given up).
Problems with a Cover 4
You're sacrificing all that yardage in the underneath. If your guys aren't great tacklers you can cough up some big plays. If you go into the Cover 4 too early a team can march down the field with you giving up all the underneath stuff until you're staring down the barrel of a gun. (Thus the expression "The only thing a prevent defense prevents is a win"). There's not much difference in the way a 3-4 scheme uses a Cover 4 compared to a 4-3 scheme because the Cover 4 is a pretty rare thing.
Things to consider before using a Cover 4
1. How are your DBs? Most teams don't carry enough quality DBs to be able to really play much in the way of Cover 4
2. How is your pass rush? If you can get a half-way decent pass rush with 3 down linemen a Cover 4 might work.
3. What kind of QB are you facing? If it's one who likes to scramble a Cover 4 is great because it blankets his receivers. If it's a guy who can hit the out route successfully a Cover 4 can get torched because they can complete long passes along the sidelines that stop the clock.
Next week we'll be taking a look at special teams, specifically kick returns and punt returns.