The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= i\\[1 em]x_2 &= -i\\[1 em]x_3 &= 2+2i\\[1 em]x_4 &= 2-2i \end{aligned} $$Polynomial $ x^4-4x^3+9x^2-4x+8 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.