An Etymological Dictionary of Astronomy and Astrophysics

Etymology (EN): O.E. feower, from P.Gmc. *petwor- (cf. O.S. fiwar, Du. and Ger. vier, O.N. fjorir, Dan. fire, Sw. fyra), cognate with Pers. cahâr, as below, from PIE *qwetwor.

Etymology (PE): Cahâr, variant câr, from Mid.Pers. cahâr; Av. caθwarô, catur-; cf. Skt. catvārah;
Gk. tessares; cognate with L. quattuor; E. four, as above.