also: 4-3 · base 4-3
Four down linemen, three linebackers, four defensive backs. The structure the 4-2-5 is usually defined against.
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A defensive structure with four down linemen and three linebackers, which puts seven bodies near the line before the safeties are counted. The extra linebacker is a run defender first, so the front is heavier against the run and asks more of the secondary against spread formations.
4-3, base 4-34-2-5 defense, 3-4 defenseA hybrid version in which the strongside linebacker is a smaller, faster, more coverage-capable player while the defense keeps the 4-3 name. In practice that player is often a linebacker and defensive back hybrid: a safety-sized body asked to fit a run and cover a slot, which is the same brief a nickel gets. Worth understanding as a response rather than a design, because pulling that linebacker out toward a slot is usually something a spread offense does TO a 4-3, and the standard answers are to substitute a defensive back for him, at which point it is nickel personnel rather than a 4-3, or to walk out a different linebacker instead.
Whether the result is close to a 4-2-5 depends on which taxonomy you use, and both are current. Counted by bodies on the field it is nearly the same defense. Counted by run-fit responsibility some coaches treat the 4-3 as a seven-man front and the 4-2-5 as an eight-man one, which makes them different rather than adjacent. Neither reading is wrong and a coach will meet both.
4-3 hybrid, hybrid Sam4-2-5 defense, nickel personnelAt youth and lower high school levels the third linebacker is usually the easiest position to fill, which is part of why the 4-3 stays common there long after higher levels have moved on.