In football, the 4-3 Front is a base defensive strategy that uses four defensive linemen (the "4") and three linebackers (the "3"). The 4-3 and 3-4 Fronts are the most common base defenses in the modern game.
The 4-3 has two main variations: the "Over" and the "Under". These two strategies differ in how the defensive line and linebackers line up, with a nose guard replacing a defensive tackle for either set.
The typical four-man defensive front consists of two defensive tackles and two defensive ends. In an "over" set, the nose guard will cover the weak-side gap between the center and guard (the "A" gap), while the defensive tackle will take the strong side B gap (in between the offensive guard and tackle).
The linebackers are also a crucial part of a successful 4-3 defense. There are three; one inside linebacker (aka the MLB or "Mike") and two outside linebackers. Each outside linebacker (or OLB) has a specific name; the strong-side OLB is "Sam" and the weak-side OLB is "Will".
In a 4-3 Over Front, the linebackers align in a "Tan-Zero-Tan" formation, which is what a typical 4-3 would be expected to look like.
In the Under, though, the Sam linebacker pushes up to the line of scrimmage to contain around the tight end. This often leads to what looks like a five-man defensive front, although the linebacker will still be in a 2-point stance (standing position).
In the 4-3 Front, the four remaining defensive backs will almost always be two cornerbacks and two safeties. The defensive backs don't have specific jobs inherent to the 4-3 like linebackers or defensive linemen do, but without them play will certainly break down for the defense.