I'm interested in the case no element of G has order 4. How do you prove that G(any arbitrary group of order 4) is abelian?

I realize that order of any element of G has to be either 1 (identity) or 2 (not identity) based on Lagrange's theorem.

Can't figure out the exact step of algebraic manipulation to go from ab to ba to show that ab=ba.

Thanks.