\[ a y^{(4)}(x)-f(x)+y^{(5)}(x)=0 \] ✓ Mathematica : cpu = 0.0874491 (sec), leaf count = 92
\[\left \{\left \{y(x)\to \int _1^x\int _1^{K[5]}\int _1^{K[4]}\int _1^{K[3]}\left (e^{-a K[2]} c_1+e^{-a K[2]} \int _1^{K[2]}e^{a K[1]} f(K[1])dK[1]\right )dK[2]dK[3]dK[4]dK[5]+c_5 x^3+c_4 x^2+c_3 x+c_2\right \}\right \}\] ✓ Maple : cpu = 0.314 (sec), leaf count = 40
\[ \left \{ y \left ( x \right ) ={\frac {{\it \_C3}\,{x}^{2}}{2}}+{\frac {{\it \_C2}\,{x}^{3}}{6}}+{\frac {{{\rm e}^{-ax}}{\it \_C1}}{{a}^{4}}}+{\frac {f{x}^{4}}{24\,a}}+{\it \_C4}\,x+{\it \_C5} \right \} \]