\[ \boxed { f{\frac {{\rm d}^{4}}{{\rm d}{x}^{4}}}y \left ( x \right ) =0} \]
Mathematica: cpu = 1.265661 (sec), leaf count = 44 \[ \left \{\left \{y(x)\to \int _1^x \left (\int _1^{K[2]} \left (\frac {c_1}{f(K[1])}+\frac {c_2 K[1]}{f(K[1])}\right ) \, dK[1]\right ) \, dK[2]+c_4 x+c_3\right \}\right \} \]
Maple: cpu = 0.016 (sec), leaf count = 21 \[ \left \{ y \left ( x \right ) ={\frac {{\it \_C1}\,{x}^{3}}{6}}+{\frac { {\it \_C2}\,{x}^{2}}{2}}+{\it \_C3}\,x+{\it \_C4} \right \} \]