The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 1.5723\\[1 em]x_2 &= -1.5723\\[1 em]x_3 &= 2.544i\\[1 em]x_4 &= -2.544i \end{aligned} $$Polynomial $ x^4+4x^2-16 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.