4. One morning at 9:00, three people, A, B, and C, set off simultaneously and drove south along the Jingzhu Expressway. At the start, C was $180 \mathrm{~km}$ ahead of A, and B was $100 \mathrm{~km}$ behind C. The speeds of A, B, and C were $120 \mathrm{~km} / \mathrm{h}$, $100 \mathrm{~km} / \mathrm{h}$, and $80 \mathrm{~km} / \mathrm{h}$, respectively. By afternoon $\qquad$ o'clock, the distances between A and B, and A and C were exactly the same.
4.2 Points or 1:20 PM.

Let $t$ be the hours after which the distance between A and B, C is exactly the same. Then:
(1) When B catches up with C, the distance between A and B, C is of course the same, at this time, $t=\frac{100}{100-80}=5$.
(2) When A surpasses B and is between B and C, the condition is also met. For this situation, C can be considered as stationary, the speeds of A and B are $40 \mathrm{~km} / \mathrm{h}$ and $20 \mathrm{~km} / \mathrm{h}$, respectively, then we have
$$
40 t=180-\frac{1}{2}(100-20 t) \text {. Solving this, we get } t=\frac{13}{3} \text {. }
$$