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