Quad- means four
quadrangle (n) - a four-sided enclosure, usually surrounded by buildings
quadrant (n) - one out of four equal parts of a circle; one section of a four-section coordinate grid
quadrennial (adj) - happening every four years
quadriceps (n) - a muscle group consisting of four muscles that is located along the front of the thigh
quadrilateral (n) - a plane figure in geometry that has four sides (including square, rectangle, rhombus, parallelogram, and trapezoid)
quadrilingual (adj) - the ability to speak four languages
quadrisect (v) - to cut or divide into four equal parts
quadruped (n) - an animal with four feet
quadruple (v) - to create four times as much or as many of something
quadruplet (n) - a collection of four; one of four babies born from the same mother at the same time