If someone is playing for $4 per ball and can't play a lick, it is risky to predict how they are going to score.I thought it was loser pays for each ball he has on the table.
If you are paying by the ball, it is not unreasonable to count the number of balls the winner made (8 at 8 ball, usually) minus the number the loser made, which could be from 0 to 7.
Not clear how you are supposed to score an early 8.