The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 6.1685\\[1 em]x_2 &= -6.1685\\[1 em]x_3 &= 6.4846i\\[1 em]x_4 &= -6.4846i \end{aligned} $$Polynomial $ x^4+4x^2-1600 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.