4. If the orthocenter of $\triangle O A B$ is exactly the focus of the parabola $y^{2}=4 x$, where $O$ is the origin, $A, B$ are on the parabola, then the area $S$ of $\triangle O A B$ is $\qquad$ .
$F(1,0)$ is the orthocenter of $\triangle O A B$, then $A B \perp x$ axis. Let $A\left(t^{2}, 2 t\right), B\left(t^{2},-2 t\right)(t>0)$, by $\overrightarrow{A F} \cdot \overrightarrow{O B}=\left(1-t^{2},-2 t\right) \cdot\left(t^{2},-2 t\right)=0 \Rightarrow 5 t^{2}-t^{4}=0 \Rightarrow t=\sqrt{5}$. Therefore, $S=\frac{1}{2} \cdot 4 \sqrt{5} \cdot 5=10 \sqrt{5}$.