4. As shown in the figure, in rectangle $\mathrm{ABCD}$, $\mathrm{AB}: \mathrm{BC}=5: 4$. The first ant, starting from point $\mathrm{A}$, moves in the direction $A \rightarrow B \rightarrow C \rightarrow D \rightarrow A$, while the second ant, starting from point $\mathrm{C}$, moves in the direction $C \rightarrow B \rightarrow A \rightarrow D \rightarrow C$. Both ants start at the same time and crawl along the edges of the rectangle. If the two ants meet for the first time at point B, then the two ants will meet for the second time on the ( ) edge.
(A) $\mathrm{AB}$
(B) $\mathrm{BC}$
(C) $C D$
4. Choose D
Solution: In rectangle $\mathrm{ABCD}$, $\mathrm{AB}: \mathrm{BC}=5: 4$. Divide $\mathrm{AB}$ and $\mathrm{CD}$ into 5 equal parts, and $\mathrm{BC}$ and $\mathrm{DA}$ into 4 equal parts. Let the length of each part be $\mathrm{a}$. Since the two ants first meet at point $\mathrm{B}$, the first ant walks $5a$, and the second ant walks $4a$. Next, when the first ant walks from $\mathrm{B}$ to point $\mathrm{E}$, the second ant walks from $\mathrm{B}$ to point $\mathrm{F}$. Then, when the first ant walks to point $\mathrm{G}$, the second ant also walks from $\mathrm{F}$ to $\mathrm{G}$, where the two ants meet for the second time on the $\mathrm{DA}$ side.