Based on Ken's predictions, if the average low temperature in New York City for August is expected to be 20°C, and the temperature is projected to drop by 4°C in September, we can calculate the expected average low temperature for September as follows:
Average low temperature in August: 20°C
Temperature drop in September: -4°C
Average low temperature in September = 20°C - 4°C = 16°C
So, Ken predicts that the average low temperature in New York City will be 16°C in September.