source 1987 Montgomery "Speeding the Pollard and elliptic curve methods of factorization", page 261, fifth display
compute X4 = Z1((X3-Z3)(X2+Z2)+(X3+Z3)(X2-Z2))^2
compute Z4 = X1((X3-Z3)(X2+Z2)-(X3+Z3)(X2-Z2))^2
