The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.7152+0.7218i\\[1 em]x_2 &= 0.7152-0.7218i\\[1 em]x_3 &= -1.7152+0.9656i\\[1 em]x_4 &= -1.7152-0.9656i \end{aligned} $$Polynomial $ x^4+2x^3-2x+4 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.