The integral \( \int_{0}^{\pi} \frac{8x \, dx}{4\cos^{2} x + \sin^{2} x} \) is equal to:
\( 4\pi^{2} \)
\( \pi^{2} \)
\( 2\pi^{2} \)
\( \frac{3\pi^{2}}{2} \)