The 4-3 Under is an alignment from a four-man defensive front that puts the 3-technique away from the strength of the offense. The Under is predominantly a one-gap system that plugs up the run and creates advantageous pass-rush situations.
Why Use the 4-3 Under
The 4-3 Under helps cover the weak side linebackers to keep him free to make plays. With the 3-technique shifted to the weak side of the formation, that makes it difficult for the offensive linemen on that side to climb up to the linebacker since they have a player directly over them. That helps enable the weak linebacker to more quickly scrape to any strong-side run action.
To the strong side of the formation, the 1-technique and strong side linebacker are usually better at taking on and shedding blocks. This also isolates the 3-technique on the guard away from any help like a tight end, fullback, or H-back. The 3-tech is usually the best pass rushing interior lineman and has a one-on-one with the guard to the weak side of the formation.
How to Identify
You can identify the 4-3 Under by the placement of the 3-technique. That 3-technique is away from the offensive strength. The other defensive tackle aligns to the strength of the formation in a nose, 1-technique, or 2i. It depends on the philosophy of the defensive coordinator.
Strengths of 4-3 Under
One-on-ones with the 3-technique and weak defensive end
By alignment, the offense has no ability to double team anyone but the nose tackle to their formational strength. That leaves both the weakside defensive end and 3-technique with one-on-ones. Those players must be able to cut through the backfield on run plays and win on their pass rushes.
No coverage limitations
The 4-3 Under doesn’t limit the types of coverage that are run behind it. It’s able to utilize everything from Cover 1 with one safety to Cover 4 with two safeties. The front remains the same with an outside linebacker removed for two-safety shells.
Weaknesses of 4-3 Under
Reliance on having a good 3-technique and a Will linebacker that can scrape
Since the Will linebacker is protected by the weakside defensive end and 3-technique, a lot of plays and tackles are going to be funneled his way. He has to be able to sift through traffic as the nose and Mike linebacker spill things back his direction.
Additionally, if you don’t have a 3-technique that can win a one-on-one, the scheme falls apart. The defense is built around isolating the 3-technique. If he can’t win, it’s a problem.
Necessity for Sam that can cover and run defend
The strongside Sam linebacker, who is sometimes played down on the line of scrimmage, has to be able to jam the tight end off the line of scrimmage and occasionally run with them in pass coverage. They also have to be strong enough force runs inside and hold up against tackles and tight ends.
The 4-3 Under isolates the weakside defensive end and 3-technique to create advantageous matchups. The front can be demanding on the Sam linebacker, but if you have guys that can win at those positions, it’s hard for offenses to offer help to their weak side offensive linemen.