Expanding the expression, we get:

-2x(x+1)(x-4)(x^2-16)
= -2x(x^3 - 4x^2 + x^2 - 4x -16x + 64)
= -2x(x^3 - 3x^2 - 20x + 64)
= -2x^4 + 6x^3 + 40x^2 - 128x