Maximum Covariance

Let $X$ and $Y$ be random variables with $\mathbb{E}\left[ X \right] = \mu_1, \mathbb{E}\left[ Y \right] = \mu_2, \var{X} = \sigma_1^2,$ and $\var{Y} = \sigma_2^2$. Find the maximum possible value of $\cov{X}{Y}$ when $\mu_1 = \mu_2 = 4$, $\sigma_1^2 = 16$, and $\sigma_2^2 = 25$.