4x^4+4x^3-9x^2-x+2
=4x^4+4x^3-8x^2-x^2-x+2
=(4x^4+4x^3-8x^2)-(x^2+x-2)
=4x^2(x^2+x-2)-(x^2+x-2)
=(4x^2-1)(x^2+x-2)
=(2x+1)(2x-1)(x-2)(x+1)