The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 1.0535\\[1 em]x_2 &= -1.0535\\[1 em]x_3 &= -2.1199\\[1 em]x_4 &= 2.1199\\[1 em]x_5 &= 1.2664i\\[1 em]x_6 &= -1.2664i \end{aligned} $$Polynomial $ x^6-4x^4-4x^2+8 $ has no rational roots that can be found using Rational Root Test, so the roots were found using Newton method.