4. Given that $P$ is a point on the ellipse $\frac{x^{2}}{25}+\frac{y^{2}}{9}=1$, $F_{1}$ is its left focus, and $Q$ is on $P F_{1}$ such that
$$
\overrightarrow{O Q}=\frac{1}{2}\left(\overrightarrow{O P}+\overrightarrow{O F_{1}}\right),|\overrightarrow{O Q}|=3 \text {. }
$$

Then the distance from point $P$ to the left directrix of the ellipse is