4. In Rt $\triangle A B C$, $C D$ is the altitude to the hypotenuse $A B$, then the sum of the inradii of the three right triangles ( $\triangle A B C, \triangle A C D$, $\triangle B C D$ ) is equal to ( ).
(A) $C D$
(B) $A C$
(C) $B C$
(D) $A B$
4. A