To calculate the drop rate, we need to determine the number of drops per minute.

First, we need to convert the 4 hours into minutes: 4 hours * 60 minutes/hour = 240 minutes

Next, we need to calculate the total number of drops needed: 500 ml * 15 drops/ml = 7500 drops

Finally, we divide the total number of drops by the total number of minutes to find the drop rate: 7500 drops / 240 minutes = 31.25 drops/minute

Therefore, the drop rate is 31.25 drops/minute.