Assuming that the equation has a polynomial solution of degree n, the highest term in x on the left hand-side is n+1, and this must be 4 for the equation to hold, thus n=3. Then, equating the coefficients of x^4 on both sides of the equation, we see that the polynomial is monic, thus it has the form y(x)=x^3+bx^2+dx+e, with b,d,e real. Then,
differentiating, substituting into the equation, and equating the coefficients of the same degree terms on both sides, we end up to the following non-linear system of equations
2b+c=0, 2d+bc-3=0, 2e+dc-2b=0, and ec-d=2. Solving the first equation for c and substituting into the other three, we obtain the system
2d-2b^2-3=0, 2e-2db-2b=0, -2eb-d=2. Then, solving the second equation for e and substituting into the third, we obtain the equation 2(d+1)b^2=-2-d. If d equals -1, this equation is impossible, thus d is different from -1 and solving for b^2 we obtain
b^2=-(d+2)/2(d+1). Then, solving 2d-2b^2-3=0 for b^2 and comparing with the last equation yields -(d+2)/2(d+1)=(2d-3)/2 which is easily solved for d to give d=+-1/sqr(2). Then, 2d-3=+-sqr(2)-3
you may also insert a formal series into the differential equations then prove that the relationship between its coefficients leads to an infinite sequence of non zero values.