Answer: To calculate the probability of getting exactly 4 donors before finding an A+ donor, we need to make a few assumptions:
Assuming each donor is independent and has an equal chance of being an A+ donor.
Assuming the probability of selecting an A+ donor is p (0 < p < 1).
The probability of not getting an A+ donor after selecting one donor is (1 - p), and the probability of getting an A+ donor for the first time after selecting four donors is (1 - p) * (1 - p) * (1 - p) * p.
Therefore, the probability of getting exactly 4 donors before finding an A+ donor is:
P(4 donors before A+) = (1 - p)^3 * p
Please note that without knowing the specific value of p, we cannot provide the exact probability.