Determining the prop grip is a factor of several things:

First, hand size and prop size. To maintain control over releasing one prop at a time requires that you be able to hold onto the remaining props, otherwise they all fly out together (not very helpful).

Second, each successive grip position should be the position that is already familiar. So, if you are releasing four from one hand, after the first ball is thrown, the remaining balls are in the already established three ball grip. Build upon those skills that have already been learned.

In the following pictures, the balls are released in the order indicated.

Basic Two Ball Grip