smc

A 25% increase means the new population is $\frac{5}{4}$ of the original population. A 25% decrease means the new population is $\frac{3}{4}$ of the original population. Thus, after four years, the population is $1\cdot \frac{5}{4}\cdot \frac{5}{4}\cdot \frac{3}{4}\cdot \frac{3}{4}=\frac{225}{256}$ times the original population. Thus, the net change is -12%, so the answer is $\boxed{-12}$.