\[ y'(x)=\frac {\frac {1}{2} x^3 \cos (2 y(x))+\frac {x^3}{2}-\frac {1}{2} \sin (2 y(x))}{x} \] ✓ Mathematica : cpu = 0.0761866 (sec), leaf count = 21
\[\left \{\left \{y(x)\to \tan ^{-1}\left (\frac {2 c_1+x^4}{4 x}\right )\right \}\right \}\] ✓ Maple : cpu = 0.665 (sec), leaf count = 17
\[ \left \{ y \left ( x \right ) =\arctan \left ( {\frac {{x}^{4}+8\,{\it \_C1}}{4\,x}} \right ) \right \} \]