Nouns

(n)quartet, quartette, quadruplet, quadruplea set of four similar things considered as a unit
(n)quadruplea quantity that is four times as great as another

Verbs

(v)quadrupleincrease fourfold“His stock earning quadrupled”

Adjectives

(s)quadruple, quadruplicate, quadruplex, fourfold, four-foldhaving four units or components“quadruple rhythm has four beats per measure”, “quadruplex wire”
(s)quadruple, fourfold, four-foldfour times as great or many“a fourfold increase in the dosage”