fouryoudaoicibaDictYouDict[four 词源字典]
four: [OE] The distant Indo-European ancestor of English four was *qwetwōr-, which also produced Latin quattuor (whence French quatre, Italian quattro, Spanish cuatro, etc), Greek téssares, Sanskrit catvāras, Russian chetvero, and Welsh pedwar. Its Germanic descendant was *petwor-, from which come German and Dutch vier, Swedish fyra, and English four.

Amongst the word’s perhaps more surprising relatives in English are carfax ‘crossroads’ (ultimately from a post-classical Latin compound which meant literally ‘four forks’), quarantine (denoting etymologically a ‘period of 40 days’), quire ‘set of four sheets of paper’, and trapeze (literally ‘four-footed’).
=> carfax, quarantine, quire, trapeze[four etymology, four origin, 英语词源]
four (n., adj.)youdaoicibaDictYouDict
Old English feower "four; four times," from Proto-Germanic *fedwor- (cognates: Old Saxon fiuwar, Old Frisian fiower, fiuwer, Frankish *fitter-, Dutch vier, Old High German fior, German vier, Old Norse fjorir, Danish fire, Swedish fyra, Gothic fidwor "four"), from PIE *kwetwer- "four" (cognates: 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; Watkins explains the -f- as being from the following number (Modern English five).

To be on all fours is from 1719; earlier on all four (14c.). Four-letter word as a euphemism for one of the short words generally regarded as offensive or objectionable is attested from 1923; four-letter man is recorded from 1920 (apparently as a euphemism for a shit). Compare Latin homo trium litterarum, literally "three-letter man," a euphemism for fur "a thief." A four-in-hand (1793) was a carriage drawn by 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, compare French Livres des Quatre Rois) contains an old 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. The four-color problem so called from 1879. The four-minute mile was attained 1954.