 The M-4 had a new longer span wing and the Post Office converted some of the M-3s order to be built as M-4s.One M-4 was also bought by Western Air Express and this was designated the M-4A.When the CAM-3 (Chicago-Dallas) route was passed to National Air Transport Inc (NAT) in October 1925, the airline originally used the Curtiss Carrier Pigeon.