CREATE TABLE table_name_4 (enrollment VARCHAR(255), ihsaa_class VARCHAR(255), primary_conference VARCHAR(255), county VARCHAR(255))
Which Enrollment has a Primary Conference of mid-hoosier, and a County of 16 decatur, and an IHSAA Class of aa?
    
SQL: SELECT enrollment FROM table_name_4 WHERE primary_conference = "mid-hoosier" AND county = "16 decatur" AND ihsaa_class = "aa"