(poker slang) A pair of fours. / (UK, prison slang) The cells located on the third floor.

A four-stroke engine. / A vehicle or machine powered by a four-stroke engine.

Of or relating to a design of internal combustion engine in which the pistons perform four strokes per engine cycle: intake, compression, power, exhaust.

(automotive) Abnormal operation of a two-stroke internal combustion engine, such that the cylinder only fires every second cycle (like a four-stroke engine). This occurs when the exhaust scavenging and inlet charging leaves a mixture in the cylinder which does not ignite, but needs a second scavenge/charge to produce an adequate mix.

(nautical) A boat, used in competitive rowing, having four rowers, each with a single oar / (rowing, especially in plural) A race in which such craft participate

having four doors

A motor car with four doors.

plural of four-poster

(physics) A four-vector that describes the change in a particle's four-momentum over time.

(physics) A generalization of momentum in four-dimensional spacetime.