The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.5661+0.4588i\\[1 em]x_2 &= 0.5661-0.4588i\\[1 em]x_3 &= -1.0661+0.8641i\\[1 em]x_4 &= -1.0661-0.8641i \end{aligned} $$Polynomial $ x^4+x^3-x+1 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.