The 4-3 Defense is undoubtedly the most popular defense in football in recent years. Most NFL and College football teams use the 4-3 Defense. A good number of High Schools use it as well.
The proliferation of the 4-3 Defense is for 3 reasons. It can be adapted to defend all modern offenses. Using a 7 man front makes your choice of coverages very flexible. The 4-3 Defense is known as a run stopping defense too.
Adapting to the Other Guys
The 4-3 Defense was originally built to stop the downhill running game of the Wishbone offense. Because of this, it is a favorite scheme to stop the Option and other 2 and 3 back offenses. Aligning your players in the 4-3 Defense puts you in the best position to stop the I, Wing-T, Split Back Veer, and of course the Wishbone.
Watching so many teams using Spread offenses today, this would seem to be a disadvantage. But the 4-3 Defense is simple to adjust. We know that you have a 7 man front (4 defensive linemen and 3 linebackers), so you have an additional player in your secondary compared to most defenses that are 8 man fronts.
Along with being able to line up in man coverage on 4 receivers, you can align one or both of your Outside Linebackers out to strengthen the coverage. The design of the defense lets you use only one true Linebacker, the middle backer. The outside linebackers can be great athletes who are smaller.
Running Multiple Coverages
All coaches should use man coverage. Most defenses have to use Cover 3 coverage. With the 4-3 defense, you can do both of these and much more.
The initial 4-3 Defense ran Cover 2, but I am more inclined to run Quarters. It is tough to run Quarters in an 8 man front defense. It is not natural. But Cover 2 and Quarters are natural matches to the 7 man front of the 4-3 Defense.
It is no problem to roll a safety down to the flats and play Cover 3 when running this defense as well. You can also use one of your talented linebackers to lock on one on one with a slot receiver, and use a Cover 1 coverage.
Stopping the Run
As I have already said, the 4-3 Defense is a excellent run stopping front. You only need two defensive linemen, your tackles, and one legit linebacker in the middle, to stop the run.
Your Defensive Ends should be great athletes. They play on the outside of the line and are not subject to double teams. The ends should use their abilities and always be running.
The Outside Linebackers are shielded by the tackles. The Mike Linebacker is reading the nearest back to him (the fullback, usually), and attacking to whichever side he reads the run to. He is a 2-gap player. Because of this, your outside linebackers sparingly have to take on a tough downhill run like a dive play.
The Mike linebacker takes care of the dive back in the option, allowing the Ends and Outside Linebackers to be responsible for the Quarterback and the safeties run to the Pitch.