From 四つ (yotsu, “four”) + 這い (hai, “crawling”, 連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 這(は)う (hau, “to crawl”)).
よつばい • (yotsubai) ←よつばひ (yotubafi)?