The Proto-Celtic word *kʷetru
Word *kʷetru
Meaning four
See also Avestan: caθβarasca ‘four’

Other words that might be of interest
*oino ‘one’, *dwei ‘two’, *dwā(w) ‘two’, *swexs ‘6, six’, *nawan ‘nine, 9’, *tri ‘three, 3’, *kʷenkʷe ‘five, 5’, *sextam ‘seven’