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, and points $A$ and $B$ are on the parabola, then $S_{\triangle O A B}=$ $\qquad$
4. $10 \sqrt{5}$.

It is known that the focus of the parabola is $F(1,0)$.
Since $F$ is the orthocenter of $\triangle O A B$, we know $O F \perp A B$.
Let $A\left(a^{2}, 2 a\right), B\left(a^{2},-2 a\right)(a>0)$. Then
$$
\begin{array}{l}
l_{O A}: a y=2 x, k_{O A}=\frac{2}{a}, k_{B F}=\frac{-2 a}{a^{2}-1} . \\
\text { Since } k_{B F} \cdot k_{O A}=-1 \Rightarrow a=\sqrt{5} \\
\Rightarrow A B=4 \sqrt{5}, h=a^{2}=5 \\
\Rightarrow S_{\triangle O A B}=10 \sqrt{5} .
\end{array}
$$