\[ f''(x) y''(x)+2 y^{(3)}(x) f'(x)+f(x) y^{(4)}(x)=0 \] ✓ Mathematica : cpu = 0.0774019 (sec), leaf count = 46
\[\left \{\left \{y(x)\to \int _1^x\int _1^{K[2]}\left (\frac {c_1}{f(K[1])}+\frac {c_2 K[1]}{f(K[1])}\right )dK[1]dK[2]+c_4 x+c_3\right \}\right \}\] ✓ Maple : cpu = 0.016 (sec), leaf count = 21
\[\left \{y \left (x \right ) = \frac {1}{6} c_{1} x^{3}+\frac {1}{2} c_{2} x^{2}+c_{3} x +c_{4}\right \}\]