This Mah is not “what” which would portend a 5th question. It is an exclamation of “How” as in “Mah Tov” & “Mah No’iem”.
This is the opinion of the Lail Shimurim haggada by Rav Yechiel Michel Halevi Epstein Author of Aruch Hashulchan.
It is said in amazement: Oh How this night is different from other nights!