4x^2y^2-(x^2+y^2-z^2)^2
=(2xy+x^2+y^2-z^2)(2xy-x^2-y^2+z^2)
=[(x+y)^2-z^2][z^2-(x-y)^2]
=(x+y+z)(x+y-z)(z-x+y)(z+x-y)