quadrennial (adj.)

1650s, "lasting four years, comprising four years;" as "happening once in four years," 1701; from quadri- + ending from biennial, etc. Correct formation would be quadriennial (compare Latin quadriennium "period of four years," which also is sometimes used in English). As a noun from 1640s. Quadrennium as "a period of four years" is by 1735. Related: Quadrennially.