4. In the Cartesian coordinate system $x 0 y$, it is known that point $A(3,-3)$, and $P$ is a point on the $y$-axis. Then the number of points $P$ that make $\triangle A O P$ an isosceles triangle is ( ).
(A) 2
(B) 3
(C) 4
(D) 5
4.C.

Considering two cases with points $A$, $O$, and $P$ as the apex of an isosceles triangle respectively