\[ y'(x)-\frac {\sqrt {1-y(x)^4}}{\sqrt {1-x^4}}=0 \] ✓ Mathematica : cpu = 0.21885 (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 = 0.017 (sec), leaf count = 51
\[ \left \{ {{\it EllipticF} \left ( x,i \right ) \sqrt {-{x}^{2}+1}\sqrt {{x}^{2}+1}{\frac {1}{\sqrt {-{x}^{4}+1}}}}-\int ^{y \left ( x \right ) }\!{\frac {1}{\sqrt {-{{\it \_a}}^{4}+1}}}{d{\it \_a}}+{\it \_C1}=0 \right \} \]