Ok, I was able to find the following

> G4Z2s1 := Simplify(subs(substtot, G4Z2Sa))*I;
Error, (in Check) found different free indices in different operands of a sum; in operand 1: [A, B, C, D], in operand 2: [4, C, D, A, B]

> Simplify(subs(substtot, op(2, G4Z2Sa)));                                                                      
lambda  Delta[X4, X3] g_[C, D] Delta[w, w] Delta[z, w] Delta[X2, z] Delta[w, z] Delta[z, X1] g_[A, B]
 

As you can see there is no index 4 in the second operand of G4Z2Sa. What's going on? Is this a bug?