4. In $\triangle A B C$, the sides opposite to $\angle A$, $\angle B$, and $\angle C$ are denoted as $a$, $b$, and $c$ respectively. If $b^{2}=a^{2}+c^{2}-a c$, and $c-a$ equals the height $h$ from vertex $A$ to side $A C$, then $\sin \frac{C-A}{2}=$ $\qquad$