Let \( \lambda x - 2y = \mu \) be a tangent to the hyperbola \( a^{2}x^{2} - y^{2} = b^{2} \). Then \( \left( \frac{\lambda}{a} \right)^{2} - \left( \frac{\mu}{b} \right)^{2} \) is equal to:
\( -2 \)
\( -4 \)
\( 2 \)
\( 4 \)