\[ -f(x)+y^{(4)}(x)+4 y(x)=0 \] ✓ Mathematica : cpu = 0.337047 (sec), leaf count = 223
\[\left \{\left \{y(x)\to e^{-x} \left (\cos (x) \int _1^x\frac {1}{8} e^{K[1]} f(K[1]) (\cos (K[1])-\sin (K[1])) \left (\cos ^2(K[1])+\sin ^2(K[1])\right )dK[1]+\sin (x) \int _1^x\frac {1}{8} e^{K[2]} f(K[2]) (\cos (K[2])+\sin (K[2])) \left (\cos ^2(K[2])+\sin ^2(K[2])\right )dK[2]+e^{2 x} \sin (x) \int _1^x\frac {1}{8} e^{-K[3]} f(K[3]) (\cos (K[3])-\sin (K[3])) \left (\cos ^2(K[3])+\sin ^2(K[3])\right )dK[3]+e^{2 x} \cos (x) \int _1^x-\frac {1}{8} e^{-K[4]} f(K[4]) (\cos (K[4])+\sin (K[4])) \left (\cos ^2(K[4])+\sin ^2(K[4])\right )dK[4]\right )+c_1 e^{-x} \cos (x)+c_4 e^x \cos (x)+c_2 e^{-x} \sin (x)+c_3 e^x \sin (x)\right \}\right \}\] ✓ Maple : cpu = 0.018 (sec), leaf count = 36
\[y \relax (x ) = \frac {f}{4}+c_{1} {\mathrm e}^{x} \cos \relax (x )+c_{2} {\mathrm e}^{x} \sin \relax (x )+c_{3} {\mathrm e}^{-x} \cos \relax (x )+c_{4} {\mathrm e}^{-x} \sin \relax (x )\]