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