"eighty, four times twenty," mid-13c., "formerly current as an ordinary numeral" [OED], from four + score (n.). Archaic by the time Lincoln used it at Gettysburg in 1863. Related: Fourscorth "eightieth."
four-flusher
fourfold
four-footed
Fourierism
four-poster
fourscore
foursome
four-square
fourteen
fourteenth
fourth