The airplane departed Dallas for Tucson at 16:56 to cruise at 18,000 feet. After reaching cruising altitude, the flight proceeded in a routine manner until approximately 19:50, at which time the crew felt a severe jolt when the no. 4 engine separated from its mountings. The nacelle caught fire which was quickly extinguished. Unable to determine the extent of the damage in the nacelle, the crew elected to make an emergency landing at the nearest field, which was at the CAA Auxiliary Field, Columbus, New Mexico. This was accomplished without further incident at 20:10. PROBABLE CAUSE: “Fatigue failure of the propeller blade resulting from circumferential indentation induced on the inside surface of the blade shank during manufacture.”