From PIE *kʷetwóres — Grimm's Law turned *kʷ into /f/, disguising its kinship with Latin 'quattuor' and Greek 'téttares.'
The cardinal number following three and preceding five; the sum of two and two.
From Old English 'fēower,' from Proto-Germanic *fedwōr, from PIE *kʷetwóres (four). The initial /f/ in the English form is a textbook demonstration of Grimm's Law: PIE *kʷ regularly became Proto-Germanic *f (more precisely *hw > *f in this environment), which is why English 'four' begins with f- while Latin 'quattuor,' Greek 'téttares,' and Sanskrit 'catvāras' preserve the original velar or palatal onsets. The numeral four is one of the most securely reconstructed PIE words, attested in virtually every branch of the family with regular sound correspondences. The PIE numeral
English 'four' and Latin 'quattuor' look nothing alike, yet they are the same word subjected to Grimm's Law: PIE *kʷ became /f/ in Germanic. This is also why 'forty' is spelled without a 'u' — it comes from Old English 'fēowertig,' and the vowel was lost, unlike in 'four' + 'teen' = 'fourteen.'