Table schema:
CREATE TABLE table_name_4 (enrollment VARCHAR, ihsaa_class VARCHAR, primary_conference VARCHAR, county VARCHAR)

Question: 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"