4x^2-(4x^2-4xy+y^2)
=4x^2-(2x-y)^2
=(2x-2x+y)(2x+2x-y)
=y(4X-y)
或者：
4x^2-(4x^2-4xy+y^2)
=4x^2-4x^2+4xy-y^2
=4xy-y^2
=y(4x-y)