\[ \boxed { 3\, \left ( {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) \right ) {\it d4y} \left ( x \right ) -5\, \left ( {\frac {{\rm d}^{3}}{{\rm d}{x}^{3}}}y \left ( x \right ) \right ) ^{2}=0} \]
Mathematica: cpu = 0.036505 (sec), leaf count = 28 \[ \left \{\left \{y(x)\to c_2 \left (-\sqrt {3 c_1+2 x}\right )+c_4 x+c_3\right \}\right \} \]
Maple: cpu = 0.203 (sec), leaf count = 30 \[ \left \{ y \left ( x \right ) =3\, \left ( {\it \_C2}+x \right ) \sqrt {6} {{\it \_C1}}^{2}\sqrt {-{\frac {{\it \_C1}}{{\it \_C2}+x}}}+{\it \_C3} \,x+{\it \_C4} \right \} \]