The airmail service pilots, the mechanics, and the officials of the Post Office knew at the start that their de Havillands were not designed to fly with heavy mail loads over long distances. They also learned lessons the hard way through mechanical failures, forced landings and tragic crashes. These experiences and their combined knowledge led them to order many necessary changes to make the DH-4 model able to transport the pilots and the mail more safely and quickly.
Some of the changes made by the Post Office Department to the DH-4 included:
| * | In 1919, the forward cockpit was converted into a cargo compartment. The pilot now sat in the rear cockpit. This moved him farther from the dangers of an engine fire. This switch helped to balance the airplane and helped to make it safe to fly with up to 500 pounds of mail. |
| * | Then the exhaust system had to be extended because the original exhaust pipe opened onto the rear cockpit and the airmail pilot's vision and breathing was being impaired. |
| * | The fuselage was strengthened by replacing the cloth, outer covering with sheets of plywood. |
| * | Larger, sturdier wheels improved the DH-4's landing gear. |
| * | Problems with the instrument panel were fixed. |
| * | In 1921, pilots were assigned their own airplanes and were allowed to make modifications for their personal needs and safety. |
| * | In 1924, the airmail service added the first lights to the wings of the de Havilland airplanes. |
| * | In 1925 radios were installed. |
