Consider a Poisson process \( \{ X(t), t \geq 0 \} \). The probability mass function of \( X(t) \) is given by
\[
f(t) = \frac{e^{-4t} (4t)^n}{n!}, \quad n = 0, 1, 2, \dots
\]
If \( C(t_1, t_2) \) is the covariance function of the Poisson process, then the value of
\[
C(5, 3) \text{(in integer) is equal to} ________.
\]