4. Three people, A, B, and C, are training in a broadcast tournament format. In each round, 2 people play a singles match, and the other 1 person is the referee. The loser of each round becomes the referee for the next round, and the previous referee challenges the winner. At the end of the half-day training, it was found that A played a total of 12 rounds, B played 21 rounds, and C was the referee for 8 rounds. Therefore, the loser of the 10th round ( ).
(A) must be A
(B) must be B
(C) must be C
(D) cannot be determined
4.A.

A total of $12+21-8=25$ games were played, with Jia serving as the referee for $25-12=13$ games. Since the same person cannot serve as the referee for two consecutive games, Jia must have been the referee for the 1st, 3rd, ..., 11th, ..., 25th games. Therefore, the loser of the 10th game was Jia.