Let \( x^* y = x^2 + y^3 \) and \( (x^ * 1) ^* 1 = x^* (1 ^* 1) \). Then a value of \( 2 \sin^{-1}\!\left(\frac{x^4 + x^2 - 2}{x^4 + x^2 + 2}\right) \) is:
\( \frac{\pi}{4} \)
\( \frac{\pi}{3} \)
\( \frac{\pi}{2} \)
\( \frac{\pi}{6} \)