Where does “τέταρτος” come from?

τέταρτος (Greek) comes from Ancient Greek τέταρτος, 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.

τέταρτος (Greek): fourth