Definitions:
a quadrangle

a quadruplet

a quadriceps muscle

a quad bike

quadraphony

(in telephony) a group of four insulated conductors twisted together, usually forming two circuits.

a radio aerial in the form of a square or rectangle broken in the middle of one side.

a traditional four-wheeled roller skate.

a small metal block in various sizes, lower than type height, used in letterpress printing for filling up short lines.

quadruple or quadraphonic.