(x2+y2−z2)2−4x2y2=(x2+y2−z2)2−(2xy)2=((x2+y2−z2)−2xy)((x2+y2−z2)+2xy)=(x2+y2−z2−2xy)(x2+y2−z2+2xy)=(x2+y2−2xy−z2)(x2+y2+2xy−z2)=((x−y)2−z2)((x+y)2−z2)=((x−y)−z)((x−y)+z)((x+y)−z)((x+y)+z)=(x−y−z)(x−y+z)(x+y−z)(x+y+z)
Hence, (x2+y2−z2)2−4x2y2=(x−y−z)(x−y+z)(x+y−z)(x+y+z).