(physics) A generalization of velocity in four-dimensional spacetime

A motor car with four doors.

(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.

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

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

plural of four-poster

plural of four-door

Alternative form of Four Horsemen of the Apocalypse

(ice hockey, US, informal) The four regional champions of the NCAA Division I tournament, one of whom will become the national champion.

plural of four-momentum