Cover 4 is a Zone concept. Each DB takes 1/4 of the field from about seven yards out to the End Zone. However, like all coverages, there are a lot of wrinkles. Some coaches refer to it as “quarter coverage”.
Since a Cover 4 has the DBs playing back in “prevent mode” (preventing any deep pass/long run), this leaves a lot of area from the Defensive End to the sideline to cover.
In a 3-4 offense (above) with a linebacker who’s great at rushing a passer, teams often move him down off the T/TE and let him loose in generating a pass rush and then dividing the other zones between the three other linebackers. This is shown in the graphic above.
If the outside linebackers are responsible for short out type passes and runs to the outside it doesn’t open the door for a lot of QB hits unless they know where the ball is going and can get to him before it’s released. However; since the QB knows he’s stuck with a short game, he’s dumping the ball quickly.
Whether 3-4 or 4-3 (BELOW) It’s dependent on the front Defensive line to get some sort of pressure, but with 5 OL guys to block them, it doesn’t always happen.
Cover 4 is made for four DL, so if the ones we have stumble at stopping the run, it puts more onus on the outside linebacker to stop the run and pass coverage.
Which is why Cover 4 is used mainly when the offense has to have a big yardage play or they’re already winning by a bunch, so a defense will give up the short passes to eat up clock. Or they show it, but then at the last second, one or both CBs move up.
One advantage besides giving DBs less area to cover, is it does allow the defense to crowd the box on an obvious run play. The SS creeps up and now there are eight guys in the QB’s face. So, it can take away the run, the deep pass, and some short passes between the seams.
The obvious vulnerability is screens, and slants. Good QBs can tear up this formation by finding the best short passes that will get good YAC (yards after catch). Also, the play action can be a demon. The defenders move up for a run and the QB zips one deep between the safeties. TD.