Sum up the average of each month to get the total 4 month seasonal average. If you just happen to have the 4 month total by year, for a number of years n, you would sum up the 4 month totals for each year and divide by the n years. In discussing, either way would be the seasonal 4 month average, no reason to discuss or mention average of an average, which is confusing.

Summer monsoon season is represented by June, July, August and September (JJAS).

• For calculating seasonal rainfall you have to take summation of JJAS.
• For calculating mean seasonal rainfall you can take average of JJAS.

I agree with Pratik Kad and suggest that for better and accurate results

if you have actual rainfall data of 4 monsoon months, add them and then calculate the average.

if you have only the average data of these months, calculate mean seasonal rainfall as suggested by Pratik

William F. Hansen ,Pratik Kad ,Sitender Malik Thankyou for your response

I would suggest to take a average of JJAS.