The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.7321\\[1 em]x_2 &= -1.268\\[1 em]x_3 &= -2.7321\\[1 em]x_4 &= -4.7321 \end{aligned} $$Polynomial $ x^4+8x^3+16x^2-12 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.