4.8 Eight people each receive a different message at the same time. They inform each other of all the messages they know by phone, with each call taking exactly 3 minutes. To ensure that everyone knows all the messages, at least ( ) minutes are needed.
(A)6
(B) 9
(C) 12
(D) 15
4. B.

It is easy to prove that 3 or 6 minutes is impossible.
The following constructs a scenario that takes 9 minutes.
Use $1,2, \cdots, 8$ to represent these eight people and the messages they initially know. In the first phone call, 1 and 2, 3 and 4, 5 and 6, 7 and 8 exchange messages, so each person knows two; in the second phone call, 1 and 3, 2 and 4, 5 and 7, 6 and 8 exchange messages, so the first four people know $1 \sim 4$, and the last four people know $5 \sim 8$; in the third phone call, 1 and 5, 2 and 6, 3 and 7, 4 and 8 exchange messages, so each person knows all eight messages, taking a total time of $3 \times 3=9$ minutes.