\[ \boxed { {\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left ( x \right ) +3\,{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) -4\,y \left ( x \right ) =0} \]
Mathematica: cpu = 0.006001 (sec), leaf count = 54 \[ \left \{\left \{y(x)\to c_3 e^x+c_1 e^{-x/2} \sin \left (\frac {\sqrt {15} x}{2}\right )+c_2 e^{-x/2} \cos \left (\frac {\sqrt {15} x}{2}\right )\right \}\right \} \]
Maple: cpu = 0.016 (sec), leaf count = 35 \[ \left \{ y \left ( x \right ) ={\it \_C1}\,{{\rm e}^{x}}+{\it \_C2}\,{ {\rm e}^{-{\frac {x}{2}}}}\sin \left ( {\frac {\sqrt {15}x}{2}} \right ) +{\it \_C3}\,{{\rm e}^{-{\frac {x}{2}}}}\cos \left ( {\frac { \sqrt {15}x}{2}} \right ) \right \} \]