(climbing) A technical climbing technique used in ice climbing to gain a respite and lessen slack and distance from the undersurface/ceiling which one is climbing underneath, which appears like a number "4" in shape, with one arm anchoring the climber to the surface while hanging upside-down, and one leg wrapped around that arm, forming a figurative "4" in shape.

(organic chemistry) An amphetamine that produces entactogenic and stimulant effects, used recreationally.

Abbreviation of 4-fluoroamphetamine.

Alternative form of 420

Initialism of dipeptidyl peptidase 4., inhibitors of which are the gliptins

(music) A meter of five quarter notes per measure.

Initialism of jet propellant 4., a wide-cut fuel designed by the U.S. military and formerly used in colder environments.

Alternative spelling of grade four

plural of 4-L

Under the Whyte notation system, a Garratt steam locomotive that is articulated in three parts, with a boiler mounted on the centre frame, and two steam engines mounted in opposite directions on separate frames, one on each end of the boiler, and each engine having a 2-6-4 wheel configuration.