Origins
The English word 'four' descends from Old English 'fēower,' from Proto-Germanic *fedwōr, ultimately from the Proto-Indo-European numeral *kʷetwóres. The relationship between English 'four' and its Latin cognate 'quattuor' is one of the most instructive examples of how systematic sound changes can render cognate words unrecognizable without historical analysis.
The key to understanding the connection is Grimm's Law, the set of consonant shifts that separated Proto-Germanic from the other Indo-European branches. PIE *kʷ (a labiovelar stop, pronounced roughly like a 'k' with simultaneous lip rounding) became *f in Proto-Germanic in certain phonological environments. So PIE *kʷetwóres became Proto-Germanic *fedwōr, while Latin preserved the original labiovelar in 'quattuor.' Greek shows a different development: the labiovelar became a dental stop, yielding 'téttares' (Attic) or 'téssares' (Ionic). Sanskrit 'catvāras' reflects yet another treatment of the labiovelar.
The Old English form 'fēower' was disyllabic, with two distinct syllables. During the Middle English period, the word was reduced to a single syllable, with forms like 'four,' 'fower,' and 'foure' all attested. The modern spelling 'four' was standardized with the 'ou' digraph representing the historical long vowel.
Old English Period
A curious orthographic puzzle involves the pair 'four' and 'forty.' One might expect 'forty' to be spelled 'fourty,' but it is not. The reason is historical: 'forty' derives from Old English 'fēowertig,' where the unstressed first vowel was reduced and eventually lost, while 'fourteen' (from 'fēowertīene') preserved it. The distinction has been maintained in spelling ever since, and 'fourty' is considered a common misspelling.
The PIE root *kʷetwóres produced a vast family of 'four' words in English through Latin and Greek borrowings. From Latin 'quattuor' came 'quarter' (a fourth part), 'quartet,' 'quadrant,' 'quadruple,' 'quadrilateral,' and 'squad' (via Italian 'squadra,' originally a square formation of soldiers). The Latin ordinal 'quartus' (fourth) gave 'quart.' From Greek 'téttares' came 'tetrad,' 'tetrahedron,' and the prefix 'tetra-.' The word 'square' itself ultimately derives from Latin 'exquadrāre' (to make square), from 'quadra' (a square), from 'quattuor.'
The numeral four held special significance in many ancient cultures. The Pythagoreans venerated the 'tetraktys,' the triangular arrangement of ten dots in four rows, as a symbol of cosmic harmony. Many ancient cosmologies organized the world into four elements (earth, water, air, fire), four cardinal directions, and four seasons. Whether these cultural patterns reflect something cognitively special about the number four — perhaps related to the four-fold symmetry of the human visual field or the cross-shape formed by the body's axes — or are simply arbitrary traditions is a matter of ongoing debate.
Later History
Linguistically, 'four' marks an important boundary in Indo-European grammar. In PIE, the numerals one through four were fully inflected for gender and case, agreeing with their nouns like adjectives. The numerals five and above were originally uninflected nouns. This grammatical boundary between 'low' and 'high' numerals may reflect a cognitive threshold: small quantities (up to about four) are perceived directly and treated as qualities of the noun group, while larger quantities are abstract concepts requiring a separate noun.
The phonological development from Old English 'fēower' /feːower/ to Modern English 'four' /fɔːɹ/ involved the loss of the medial /w/ and the simplification of the diphthong. Different dialects handled this differently: some Northern English dialects preserve 'fower' to this day. The modern Received Pronunciation /fɔːɹ/ and General American /fɔːɹ/ both show the influence of the following /r/ on the vowel quality.