\[ \boxed { {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) =-{\frac {y \left ( x \right ) }{ \left ( ax+b \right ) ^{4}}}=0} \]
Mathematica: cpu = 0.160520 (sec), leaf count = 78 \[ \left \{\left \{y(x)\to c_1 e^{\frac {\sqrt {-\frac {1}{a^4}} a}{a x+b}} (a x+b)+\frac {c_2 e^{-\frac {\sqrt {-\frac {1}{a^4}} a}{a x+b}} (a x+b)}{2 \sqrt {-\frac {1}{a^4}} a^2}\right \}\right \} \]
Maple: cpu = 0.063 (sec), leaf count = 43 \[ \left \{ y \left ( x \right ) ={\it \_C1}\, \left ( ax+b \right ) \sin \left ( {\frac {1}{a \left ( ax+b \right ) }} \right ) +{\it \_C2}\, \left ( ax+b \right ) \cos \left ( {\frac {1}{a \left ( ax+b \right ) }} \right ) \right \} \]