The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.2862\\[1 em]x_2 &= 1.0472\\[1 em]x_3 &= -0.6667+1.2093i\\[1 em]x_4 &= -0.6667-1.2093i \end{aligned} $$Polynomial $ 7x^4+3x^2-15x+4 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.