Adam Clarke on Acts 21:23

We have four men which have a vow - From the shaving of the head, mentioned immediately after, it is evident that the four men in question were under the vow of Nazariteship; and that the days of their vow were nearly at an end, as they were about to shave their heads; for, during the time of the Nazariteship, the hair was permitted to grow, and only shaven off at the termination of the vow. Among the Jews, it was common to make vows to God on extraordinary occasions; and that of the Nazarite appears to have been one of the most common; and it was permitted by their law for any person to perform this vow by proxy. See the law produced in my note on Num 6:21 (note). "It was also customary for the richer sort to bestow their charity on the poorer sort for this purpose; for Josephus, Ant. lib. xix. cap. 6, sec. 1, observes that Agrippa, on his being advanced from a prison to a throne, by the Emperor Claudius, came to Jerusalem; and there, among other instances of his religious thankfulness shown in the temple, Ναζαραιων ξυρασθαι διεταξε μαλα συχνους, he ordered very many Nazarites to be shaven, he furnishing them with money for the expenses of that, and of the sacrifices necessarily attending it." See Bp. Pearce.