On August 28, 1998, at 1845 central daylight time (cdt), an Air Tractor AT-401, N4543F, piloted by a commercial pilot, was destroyed during a collision with the ground while maneuvering for a forced landing. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 137 aerial application flight was not operating on a flight plan. The pilot reported no injuries. The flight departed a restricted landing area near Norcross, Minnesota, at 1840 cdt. The pilot reported a partial loss after completing an aerial application swath run. He said the airplane "...did not want to climb and began to settle down." He said he ejected his chemicals and was maneuvering for the forced landing when the right wing and landing gear struck the ground. According to the pilot, the airplane cartwheeled and rolled over the ground before coming to a stop inverted. The pilot said witnesses at the restricted landing area told him the engine didn't sound normal during the takeoff and climb out. The on-scene investigation revealed the airplane was at gross weight when it had taken off about 5-minutes earlier. An examination of the engine revealed compression on all cylinders except number 4. The Federal Aviation Administration Principal Maintenance Inspector (PMI) who examined the engine said the number 4 cylinder walls and piston rings were badly worn. He said there was no debris on the cylinder's exhaust valve seat or valve face. The PMI said the engine logbooks showed it had been operated 70-hours beyond the manufacturer's recommended TBO. According to the PMI, the engine logbook showed the compression on all the cylinders were within the manufacturer's acceptable range during the April 1998 annual inspection.