Where does “τετραήμερος” come from?

τετραήμερος (Greek) comes from Greek ημέρα, from Ancient Greek Ἡμέρα, from Ancient Greek ἦμαρ, from Proto-Hellenic ā́mər, from Proto-Indo-European h₂eh₃mr̥, from Proto-Indo-European h₂eh₃- — to be hot; to burn.

τετραήμερος (Greek): four-day lasting four days