The average price for gas in Massachusetts is $3.599 a gallon in the latest weekly AAA survey, down 4 cents from the previous week’s average, AAA Southern New England said Monday. The Massachusetts average is down 13 cents over the past month, AAA Southern New England added. The current national average for gas is $3.63. A year ago at this time, the Massachusetts average price was $3.79. AAA’s weekly survey focuses on prices for self-serve, regular unleaded gas.Chris Reidy can be reached at email@example.com.