\[ y'(x)=\frac {1}{2} \sqrt {a} \left (2 \sqrt {a x^4+8 y(x)}-\sqrt {a} x^3+2 x^3 \sqrt {a x^4+8 y(x)}+2 x^2 \sqrt {a x^4+8 y(x)}\right ) \] ✓ Mathematica : cpu = 0.556973 (sec), leaf count = 64
\[\left \{\left \{y(x)\to \frac {1}{72} a \left (\left (87-72 c_1\right ) x^4-96 c_1 x^3-288 c_1 x+144 c_1^2+9 x^8+24 x^7+16 x^6+72 x^5+144 x^2\right )\right \}\right \}\]
✓ Maple : cpu = 0.568 (sec), leaf count = 40
\[ \left \{ {\frac {1}{4}\sqrt {a{x}^{4}+8\,y \left ( x \right ) }}+{\frac {-3\,{x}^{4}-4\,{x}^{3}-12\,x}{12}\sqrt {a}}-{\it \_C1}=0 \right \} \]