$\text{If } \int_{0}^{\pi} \frac{5^{\cos x} (1 + \cos x \cos 3x + \cos^2 x + \cos^3 x \cos 3x) dx}{1 + 5^{\cos x}} = \frac{k\pi}{16}, \text{ then } k \text{ is equal to } \_\_\_\_\_\_\_\_.$