4. Given in $\triangle A B C$, $\angle C=90^{\circ}, A C=4, B C=3$. $C D$ is the altitude on $A B$, $O_{1} 、 O_{2}$ are the incenters of $\triangle A C D 、 \triangle B C D$ respectively. Then $\mathrm{O}_{1} \mathrm{O}_{2}=$ $\qquad$ .
$4 \cdot \sqrt{2}$