quadrennial (adj.)

before vowels quad- (before -p- often quadru-, from an older form in Latin), word-forming element used in compounds of Latin origin and meaning "four, four times, having four, consisting of four," from Latin quadri-, which is related to quattor "four" (from PIE root *kwetwer- "four").

1620s, "lasting for two years;" 1750, "occurring every two years," from Latin biennium "two-year period," from bi- "two" (see bi-) + annus "year" (see annual (adj.)). The vowel change is "due to the Latin phonetic law according to which the unaccented and closed radical syllable of the second element of compounds, original -ă- becomes -ĕ-" [Klein]. The noun meaning "a biennial plant" (which requires two seasons of growth to produce flowers and fruit and dies the next) is attested by 1770. Related: Biennially.