To find the average amount of flour used each day, we can divide the total amount of flour by the number of days.
The total amount of flour is 15 pounds, and it is used over 4 days.
\[ \text{Average amount of flour used per day} = \frac{\text{Total amount of flour}}{\text{Number of days}} = \frac{15 \text{ pounds}}{4 \text{ days}} = 3.75 \text{ pounds per day} \]
Thus, the average amount of flour in the bag changes by 3.75 pounds each day.