4. As shown in Figure $1, A$ is on $DE$, $F$ is on $AB$, and $AC=CE$, $\angle 1=\angle 2=\angle 3$. Then the length of $DE$ is equal to ( ).
(A) $DC$
(B) $BC$
(C) $AB$
(D) $AE+AC$
4. (C).

Since $\angle 1=\angle 3$, therefore, $\angle D=\angle B$.
Since $\angle 1=\angle 2$, hence $\angle B C A=\angle D C E$.
Also, $A C=C E$, so, $\triangle D C E \cong \triangle B C A$.
Thus, $D E=A B$.