As I understand the question, the 1st, 2nd, and 3rd customers do not make a deposit, and 4th one makes a deposit. If it is the fifth customer to make a deposit, then you can change accordingly.
Probability that a customer makes a deposit = 25% = 1/4.
Probability that a customer does not make a deposit = 3/4
We assume that one customer making a deposit is not dependent on another making a deposit.
Probability needed = 3/4 * 3/4 * 3/4 * 1/4 = 27/256