To find out how many dog biscuits Katy's 4 dogs will eat in 45 days, we first determine how many biscuits they eat in one day.
Since Katy's 4 dogs eat a total of 16 dog biscuits every day, over 45 days, they will eat:
\[ 16 \text{ biscuits/day} \times 45 \text{ days} = 720 \text{ biscuits} \]
Therefore, the dogs will eat 720 biscuits in 45 days.