4. As shown in Figure 1, in $\triangle A B C$, $D$ is a point on side $A C$. Among the following four situations, the one where $\triangle A B D \backsim \triangle A C B$ does not necessarily hold is ( ).
(A) $A D \cdot B C=A B \cdot B D$
(B) $A B^{2}=A D \cdot A C$
(C) $\angle A B D=\angle A C B$
(D) $A B \cdot B C=A C$
$\cdot B D$