4. Let $0<a_{1} \leqslant a_{2} \leqslant \cdots \leqslant a_{n}<\pi, a_{1}+a_{2}+\cdots+a_{n}=A$, find the maximum value of $\sin a_{1}+$ $\sin a_{2}+\cdots+\sin a_{n}$.
4. By the method similar to the above, the maximum value of $\sin a_{1}+\sin a_{2}+\cdots+\sin a_{n}$ can be found to be $n \sin \frac{A}{n}$.