The solo pilot was doing stop and go landings and takeoffs in the tailwheel-equipped airplane to become current. During the fourth landing, the pilot lost directional control, and the airplane ground-looped. During the ground-loop, the pilot applied excessive brake, and the airplane nosed over. The airplane sustained substantial damage to the vertical stabilizer and the rudder. The pilot reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.