Looks OK to me. Remember I34 is the current at the source of both M3 and M3 (ie total current through M3 and M4) and not the current through each transistor.
I thats the case then it would make sense. However then shouldn't gm34=√(knSnI_34) since I_34 is the total current through M3 and M4? I think I am confused about the notation.