Suppose that $x$ is directly proportional to $y$. Let $x_1, x_2$ be two values of $x$ such that $\frac{x_1}{x_2} = 4$. Let their corresponding values be
$y_1$, $y_2$ respectively. If at least one of $y_1, y_2$ is nonzero, find $\frac{y_2}{y_1}$.