English word tetra- comes from Proto-Indo-European *kʷétesres, and later Proto-Hellenic *kʷétwores (Four.)

Detailed word origin of tetra-

Dictionary entry	Language	Definition
*kʷétesres	Proto-Indo-European (ine-pro)
*kʷétwores	Proto-Hellenic (grk-pro)	Four.
τέσσαρες	Ancient Greek (grc)
tetra-	English (eng)	Four.