The 4-3 defense in football is a defensive alignment strategy consisting of four defensive linemen on the line of scrimmage and three linebackers in the defensive backfield. The 4-3 defense is used primarily as a defense against running plays but can be versatile and guard passing plays as well. The four defensive linemen on the line of scrimmage are meant to deter any attempts at running the ball down the middle while the linebackers are to be versatile and defend the runners and passes. The other four players on the field are defensive backs, usually two cornerbacks and two safeties.
When the 4-3 defense is used, one or two of the linebackers often begin by lining up at the line of scrimmage to bluff a blitz, and then they back up a few yards once the ball is snapped to defend the pass or prepare for the run. Defensive linemen may also back out into coverage, in which the 4-3 defense turns into the 3-4 defense.
The 4-3 defense is probably the most used defense in all of football, alongside the 3-4 defense. It's a very versatile defensive setup that can counter just about any offensive alignment in the game. There are enough linemen to break through the line and pressure the quarterback while also stuffing the line of scrimmage. Because of this, the linebackers can take a more relaxed position of defending the pass instead of having to send more pressure at the quarterback.
The best way to get around a 4-3 defense on offense is to pass the ball and run play-action passes. There are too many players in the box on the defense, so running it would be a mistake unless the offense is calling the defense bluff and they're actually running a 3-4 defense in disguise.