4. In $\triangle A B C$, $M=\sin A+\sin B+$ $\sin C, N=\cos \frac{A}{2}+\cos \frac{B}{2}+\cos \frac{C}{2}$. The size relationship between $M$ and $N$ is ( ).
(A) $M=N$
(B) $M \leqslant N$
(C) $M \geqslant N$
(D) Cannot be determined
4.B.

In $\triangle A B C$,
$$
\sin A+\sin B=2 \cos \frac{C}{2} \cdot \cos \frac{A-B}{2} \leqslant 2 \cos \frac{C}{2} \text{. }
$$

Similarly, $\sin B+\sin C \leqslant 2 \cos \frac{A}{2}$,
$$
\sin C+\sin A \leqslant 2 \cos \frac{B}{2} \text{. }
$$

Adding the three inequalities, we get
$$
\sin A+\sin B+\sin C \leqslant \cos \frac{A}{2}+\cos \frac{B}{2}+\cos \frac{C}{2} \text{. }
$$