The points z1,z2,z3 and z4 in the complex plane are the vertices of a parallelogram taken in order if and only if
z1 - z2 = z4 - z3
Consider the above parallelogram. The meeting point of the diagonals is the midpoint of the diagonals.
The diagonals bisect each other.
⇒z1+z22 = z2+z42
⇒ z1 - z2 = z4 - z3