also: 4-2-5 · nickel base
Four down linemen, two linebackers, five defensive backs. A front built for a field with receivers spread across it rather than backs stacked behind the ball.
A defensive structure that trades a linebacker for a fifth defensive back. The two remaining linebackers handle the interior, and the extra defender aligns outside the box across from a slot receiver, close enough to help against the run and deep enough not to be an easy read. Against a spread offense this is a base defense rather than a substitution package, which is the difference between the 4-2-5 and playing nickel out of a 4-3.
4-2-5, five defensive back base4-3 defense, nickel packageThe practical reason this spread through high school football is roster shape, not scheme fashion. A fifth defensive back is easier to find than a fourth linebacker who can cover, and a smaller faster player is easier to field at most schools than a bigger one.