✓ Mathematica : cpu = 0.105738 (sec), leaf count = 84
DSolve[f[x]*y[x] + Derivative[1][y][x] + f[x]*Derivative[2][y][x] + Derivative[3][y][x] == 0,y[x],x]
✓ Maple : cpu = 0.161 (sec), leaf count = 36
dsolve(diff(diff(diff(y(x),x),x),x)+f(x)*diff(diff(y(x),x),x)+diff(y(x),x)+f(x)*y(x)=0,y(x))