The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.809+0.5878i\\[1 em]x_2 &= 0.809-0.5878i\\[1 em]x_3 &= -0.309+0.9511i\\[1 em]x_4 &= -0.309-0.9511i \end{aligned} $$Polynomial $ -x^4+x^3-x^2+x-1 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.