The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.8167\\[1 em]x_2 &= -1.8707\\[1 em]x_3 &= 2.527+1.212i\\[1 em]x_4 &= 2.527-1.212i \end{aligned} $$Polynomial $ x^4-4x^3+x^2+16x-12 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.