As far as I understand, you want to change E1, E2, E3 and E4 respectively with combinations of -1 and 1. So the following code does that:
syms z(x,y) E1 E2 E3 E4
A=perms([-1 1 -1 1]);
After this point, you can proceed. If you have further question, we can discuss from here. Hope this gives the idea.