Where does “quaternus” come from?

quaternus (Latin) comes from Latin quater, from Latin quattŭor, from Portuguese quarto, from Old Portuguese quarto, from Latin quārtus, from Proto-Indo-European kʷeturtos, from Proto-Indo-European kʷetwóres, from Proto-Indo-European kʷe- — and.

quaternus (Latin): four each; four at a time