One of the compound interest formula is: F = A(1 + x )t
where F = final amount
A = initial amount
x = interest rate, as a decimal
t = number of years
For this problem, if the inital population = A, then the final population will be 0.96A (because it decreased by 4%).
So: 0.96A = A(1 + x)12
Divide both sides by A: 0.96 = (1 + x)12
Find the 12th root of 0.96: 0.9966039468 = 1 + x
Subtract 1: x = -0.003396 (the '-' indicates that it is decreasing)