To find the unit rate of how long it took Carla to run one mile, we need to divide the total time it took her to run 4 miles by the number of miles.
Carla's total time to run 4 miles is 36 minutes.
To find the time per mile, we divide 36 minutes by 4 miles:
\[ \text{Time per mile} = \frac{36 \text{ minutes}}{4 \text{ miles}} = 9 \text{ minutes per mile} \]
So, it took Carla 9 minutes to run one mile.