4. In a square with side length 3, randomly select $n$ points, among which $m$ points are less than 1 unit away from the vertices of the square. Then, the experimental value of $\pi$ obtained by random simulation is $\qquad$ .
The points inside a square that are less than 1 unit away from a vertex form a spherical surface with a radius of 1, then we have $\frac{m}{n}=\frac{\pi}{9} \Rightarrow \pi=\frac{9 m}{n}$.