The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 0.7321\\[1 em]x_2 &= -0.5486\\[1 em]x_3 &= 1.2153\\[1 em]x_4 &= -2.7321 \end{aligned} $$Polynomial $ 3x^4+4x^3-12x^2+4 $ has no rational roots that can be found using Rational Root Test, so the roots were found using quartic formulas.