NEW YORK — Four people, including an 8-year-old boy, were wounded when a gunman on a dirt bike opened fire on a group enjoying a Brooklyn cookout, police said Sunday. The small barbecue was underway on Quincy Street near Stuyvesant Avenue, outside NYCHA’s Stuyvesant Gardens Houses in Bedford-Stuyvesant, when two men on a dirt bike rolled up about 11:30 p.m. Saturday, cops said. One of the men pulled a gun and fired multiple rounds into the crowd before speeding off. An 8-year-old boy suffered a graze wound to his left leg. A 36-year-old woman, a 27-year-old man and a 46-year-old woman were all…