\[ \boxed { {\it d4y} \left ( x \right ) +4\,y \left ( x \right ) -f=0} \]
Mathematica: cpu = 1.373674 (sec), leaf count = 265 \[ \left \{\left \{y(x)\to e^{-x} \left (\cos (x) \int _1^x \frac {1}{8} e^{K[1]} f(K[1]) \left (-\sin ^3(K[1])+\cos ^3(K[1])-\sin (K[1]) \cos ^2(K[1])+\sin ^2(K[1]) \cos (K[1])\right ) \, dK[1]+e^{2 x} \cos (x) \int _1^x -\frac {1}{8} e^{-K[4]} f(K[4]) \left (\sin ^3(K[4])+\cos ^3(K[4])+\sin (K[4]) \cos ^2(K[4])+\sin ^2(K[4]) \cos (K[4])\right ) \, dK[4]+\sin (x) \left (\int _1^x \frac {1}{8} e^{K[2]} f(K[2]) \left (\sin ^3(K[2])+\cos ^3(K[2])+\sin (K[2]) \cos ^2(K[2])+\sin ^2(K[2]) \cos (K[2])\right ) \, dK[2]\right )+e^{2 x} \sin (x) \left (\int _1^x \frac {1}{8} e^{-K[3]} f(K[3]) \left (-\sin ^3(K[3])+\cos ^3(K[3])-\sin (K[3]) \cos ^2(K[3])+\sin ^2(K[3]) \cos (K[3])\right ) \, dK[3]\right )\right )+c_2 e^{-x} \sin (x)+c_3 e^x \sin (x)+c_1 e^{-x} \cos (x)+c_4 e^x \cos (x)\right \}\right \} \]
Maple: cpu = 0.156 (sec), leaf count = 36 \[ \left \{ y \left ( x \right ) ={\frac {f}{4}}+{\it \_C1}\,{{\rm e}^{x}} \cos \left ( x \right ) +{\it \_C2}\,{{\rm e}^{x}}\sin \left ( x \right ) +{\it \_C3}\,{{\rm e}^{-x}}\cos \left ( x \right ) +{\it \_C4}\,{{\rm e} ^{-x}}\sin \left ( x \right ) \right \} \]