4. Given $\triangle A B C$

the lengths of the three medians are 3,
4, 5. Then $S_{\triangle M B C}$ is $\qquad$
4. 8 .

As shown in Figure 4, extend GD to point $D^{\prime}$, making it twice as long.
Then the side lengths of $\triangle G D^{\prime} C$ are $\frac{2}{3}$ times the lengths of the three medians of $\triangle A B C$.
Therefore, it is a right triangle, and its area is $\frac{8}{3}$.
Additionally, the area of $\triangle G D^{\prime} C$ is equal to the area of $\triangle B G C$, and is $\frac{1}{3}$ of the area of $\triangle A B C$, hence
$$
S_{\triangle A B C}=8 .
$$