4. As shown in Figure 2, in the Cartesian coordinate system, point $A\left(\cos 110^{\circ}, \sin 110^{\circ}\right)$, $B\left(\cos 50^{\circ}, \sin 50^{\circ}\right)$. The distance from the origin $O$ to the midpoint $M$ of $A B$ is ( ).
(A) $\frac{1}{2}$
(B) $\frac{\sqrt{2}}{2}$
(C) $\frac{\sqrt{3}}{2}$
(D) 1