<image> As shown in the figure, point \( A \) lies on the graph of the inverse proportional function \( y = \frac{4}{x} \), and point \( B \) lies on the graph of the inverse proportional function \( y = \frac{k}{x} \) (where \( k \neq 0 \)). Line segment \( AB \) is parallel to the \( x \)-axis. Perpendicular lines are drawn from points \( A \) and \( B \) to the \( x \)-axis, and the feet of these perpendiculars are \( C \) and \( D \) respectively. If \( OC = \frac{1}{3}OD \), then the value of \( k \) is ______.