4. Given $I$ is the incenter of acute $\triangle A B C$, and $A_{1} 、 B_{1}$ 、 $C_{1}$ are the reflections of point $I$ over $B C 、 C A 、 A B$ respectively. If point $B$ lies on the circumcircle of $\triangle A_{1} B_{1} C_{1}$, then $\angle A B C$ equals ( ).
(A) $30^{\circ}$
(B) $45^{\circ}$
(C) $60^{\circ}$
(D) $90^{\circ}$
4. C.

Since $I A_{1}=I B_{1}=I C_{1}=2 r$ (where $r$ is the inradius of $\triangle A B C$), therefore, $I$ is the circumcenter of $\triangle A_{1} B_{1} C_{1}$.
Let the intersection of $I A_{1}$ and $B C$ be $D$. Then $I B=I A_{1}=2 I D$.
Thus, $\angle I B D=30^{\circ}$.
Similarly, $\angle I B A=30^{\circ}$.
Therefore, $\angle A B C=60^{\circ}$.