4. Given $O$ as the origin, $M$ as a point on the moving chord $AB$ of the parabola $y^{2}=2 p x$ $(p>0)$. If $O A \perp O B, O M \perp A B$, denote the area of $\triangle O A B$ as $S$, and $O M=h$. Then the range of $\frac{S}{h}$ is . $\qquad$
4. $[2 p,+\infty)$.

Let $A\left(\frac{y_{1}^{2}}{2 p}, y_{1}\right) 、 B\left(\frac{y_{2}^{2}}{2 p}, y_{2}\right)$.
Since $O A \perp O B$, we have,
$$
\begin{array}{l}
y_{1} y_{2}=-4 p^{2}, \\
|A B| \geqslant\left|y_{1}-y_{2}\right|=\sqrt{y_{1}^{2}+y_{2}^{2}+8 p^{2}} \geqslant 4 p . \\
\text { Then } \frac{S}{h}=\frac{1}{2}|A B| \geqslant 2 p .
\end{array}
$$