\[ y'(x)-\frac {\sqrt {1-y(x)^4}}{\sqrt {1-x^4}}=0 \] ✓ Mathematica : cpu = 0.185079 (sec), leaf count = 14
\[\left \{\left \{y(x)\to \text {sn}\left (\left .c_1+F\left (\left .\sin ^{-1}(x)\right |-1\right )\right |-1\right )\right \}\right \}\] ✓ Maple : cpu = 0.02 (sec), leaf count = 51
\[\frac {\sqrt {-x^{2}+1}\, \sqrt {x^{2}+1}\, \EllipticF \left (x , i\right )}{\sqrt {-x^{4}+1}}-\left (\int _{}^{y \relax (x )}\frac {1}{\sqrt {-\textit {\_a}^{4}+1}}d \textit {\_a} \right )+c_{1} = 0\]