The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.2503\\[1 em]x_2 &= 2.4303\\[1 em]x_3 &= -1.3403+2.1865i\\[1 em]x_4 &= -1.3403-2.1865i \end{aligned} $$Polynomial $ x^4-16x+4 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.