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Old English feower, from Proto-Germanic *petwor- (cf. Old Saxon fiwar, Old Frisian fiuwer, Frankish *fitter-, Dutch and German vier, Old Norse fjorir, Danish fire, Swedish fyra, Gothic fidwor "four"), from PIE *kwetwer- "four" (cf. Sanskrit catvarah, Avestan čathwaro, Persian čatvar, Greek tessares, Latin quattuor, Oscan petora, Old Church Slavonic četyre, Lithuanian keturi, Old Irish cethir, Welsh pedwar). The phonetic evolution of the Germanic forms has not been fully explained.
Slang four-eyes "person who wears glasses" first recorded 1874. Four-letter word first attested 1934; four-letter man, however, is recorded from 1923 (as a euphemism for a shit). A four-in-hand (1793) was a carriage with four horses driven by one person; in the sense of "loosely tied necktie" it is attested from 1892. To study The History of the Four Kings (1760, cf. French Livres des Quatre Rois) contains euphemistic slang phrase for "a pack of cards" from the time when card-playing was considered a wicked pastime for students. Slang 4-1-1 "essential information" (by 1993) is from the telephone number called to get customer information.