Final answer:
The new differential equation modeling the population of Cook Island (P) is P' = 4(9000 - P).
Step-by-step explanation:
The new differential equation modeling the population of Cook Island (P) can be obtained by subtracting the rate at which citizens leave the island (4) from the original logistic equation.
Therefore, the new differential equation is:
P' = 4(9000 - P)