WordNet sense 1 (one of four children born at the same time from the same pregnancy):

quadruplet

WordNet sense 2 (a muscle of the thigh that extends the leg):

quadriceps, musculus quadriceps femoris, quadriceps femoris

WordNet sense 3 ((printing) a block of type without a raised letter; used for spacing between words):

space

WordNet sense 4 (a rectangular area surrounded on all sides by buildings):

quadrangle