x^4y-x^2y^3＋x^3y^2-xy^4
=x^4y+x^3y^2-(x^2y^3+xy^4)
=x^3y(x+y)-xy^3(x+y)
=(x+y)(x^3y-xy^3)
=xy(x+y)(x^2-y^2)
=xy(x-y)(x+y)^2