Inside The Playbook: The 4-3 Under

Take a page out of Pete Carroll's playbook!

Apr 25, 2016 #Football #Play Diagrams

Today, we will take you through the evolution of the 4-3 Under defensive set, one that has evolved over two decades. The 4-3 Under base defense is an offshoot of the 4-3 stunt set that the Steelers ran in the 1970’s with their vaunted Steel Curtain defense. On Sundays, you will see the Seattle Seahawks run a variation of the 4-3 Under, and for this purpose, we will focus on the version that coach Pete Carroll ran during his time at USC.

Formation reports from Krossover Football allow coaches to sort their game film based on how the offense and defense line up, and free up coaches to scout their opponents' tendencies when in specific down and distance situations.

Coaches who schedule a demo today will receive a 1-game free trial, which includes our comprehensive ODK, tendency reports, and hit charts done for you!

Don't forget to hit us up on Twitter and let us know which plays and formations you want us to cover in this series.


Carroll first got introduced to the 4-3 Under while working as a Graduate Assistant at the University of Arkansas in 1977. That year, the now-famous defensive coach Monte Kiffin took over as Arkansas’ Defensive Coordinator and installed his version of the Steelers 4-3 Stunt. Carroll would learn under Kiffin for only the 1977 season, but the two men would link up again when Carroll got his first NFL Defensive Coordinator jobs (Buffalo Bills 1984, Minnesota Vikings 1985-1989).

Kiffin would serve as the Linebackers Coach under Carroll, as they continued to perfect the 4-3 Under. Kiffin would later take the system with him when he became Defensive Coordinator of the Tampa Bay Buccaneers in 1996. In fact, thanks to, you can view Kiffin’s 1998 playbook that featured the 4-3 Under here.

The 4-3 Under at USC

In 2001, the University of Southern California hired Pete Carroll as their Head Coach, and Carroll brought his ultra-aggressive 4-3 Under with him. One of Carroll’s primary defensive philosophies is to have the defensive line aggressively attack the offensive line, while maintaining gap control. You will see this principle every Sunday when the Seahawks take the field.

The 4-3 Under base set can be run in tandem with a number of coverage schemes, including Cover 2, Cover 3, and Man-to-Man. Below we will take a look at USC’s defensive setup called “Under Cover 1 Flex".

The play call is broken down as follows:

the 4-3 under play diagram Under: signals the defensive front alignment
In the 4-3 Under front, both defensive ends will line up in the 5-Technique, or the outside shoulder of the tackle, and be responsible for the C gaps. Where Carroll’s Seattle defense differs from Kiffin’s system is that the weak side defense has evolved into the role of the LEO, or the “Elephant”. This player has the ability to be moved all around the field and can play with his hand in the dirt or from a standing rush position.

The defensive tackle lines up in the 3-Technique (the outside shoulder of the left guard) and is responsible for the B gap, while the nose tackle will line up in the 1-Technique to the strong side of the line. The nose tackle usually plays tilted, and is responsible for both A gaps.

What makes the Under different from a typical 4-3 alignment is the position of the strong side linebacker, highlighted in the red box. Instead of being in the second level, he is up on the line of scrimmage playing the 7-Technique (or a “loose” 9-Technique as referred to in USC’s scheme) lined up against the tight end.

Cover 1-Flex: Signals the coverage of the secondary Cover 1-Flex: Signals the coverage of the secondary

This particular set calls for a Cover 1, with the strong safety playing the single deep defender. “Flex” was USC’s term for the split end side of the of the offense, so the strong safety would shade toward the split end or the X receiver. The middle and weak side linebackers shift over from their typical spot in a 4-3, to cover up for the gaps vacated by the strong side linebacker playing up on the line of scrimmage. The linebackers are responsible for the strong side B and flex side A gaps, respectively. 

The free safety steps up into the flex side C gap vacated by the weak side linebacker on running plays, and is responsible for the number two receiver on the flex side on passing plays (in this case most likely the back coming out into the flat). The corners will be matched up man-to-man on their respective receivers, with the flex side corner receiving safety help over the top.

Last, but not least, we have the player that makes the formation so different, the strong side linebacker. Since he is playing up on the line of scrimmage, his main responsibilities are to cover the tight end man-to-man on passing plays, and defend against runs to the strong side. If the tight end stays home in pass protection and the back slips out of the backfield, the strong side linebacker can also be given the responsibility to pick up the back.

This is only one way to take advantage of the 4-3 Under, as the front can be combined with a number of coverage and blitz packages to maximize the talent on the field.

Do you run this system or want to see something else featured in our playbook series? Tweet at us!

Try 1 Game for Free

Schedule A Demo
Or give us a call:
(866) 892-0777
Schedule a demo through the blog to receive:
  • An online walkthrough of how Krossover can help your team
  • A custom price quote based on your needs and budget
  • A free game breakdown
  • A Q&A session with a game film specialist