If the temperature on the first day of summer in 2002 was 79 degrees Fahrenheit and this was 4 degrees less than the temperature the previous year, we can find the temperature for the first day of summer in 2001 by adding 4 degrees to the 2002 temperature.
\[ 79 + 4 = 83 \]
Therefore, the temperature on the first day of summer in 2001 was 83 degrees Fahrenheit.