$In$ the study $of$ vacuum tubes, the equation $y^{\text{'}\text{'}}+(0\mathrm{.}9)(y^2-1)y^{\text{'}}+y=0is$ encountered. Find the Taylor polynomial $of$ degree $4$ approximating the solution with initial values $y(0)=1,y^{\text{'}}(0)=0.$

$p_4(x)=$