4. In $\triangle A B C$, it is known that $\angle B A C=100^{\circ}, A B$ $=A C, P$ is a point inside $\triangle A B C$, and $\angle P A C=$ $\angle A C P=20^{\circ}$. Find $\angle P B A$.
Prompt: In $\triangle A B C$, construct $\triangle A Q B \cong \triangle A P C$.
First, prove that $\triangle Q A P$ is an equilateral triangle, then according to criterion 1, $Q$ is the circumcenter of $\triangle A B P$.
Answer $30^{\circ}$.