A large four-engine aircraft was on a cargo flight at night when a loud bang was heard, accompanied by a loss of power from both engines on the left side. After an emergency landing, it was discovered that the propellers from both left side engines were missing. The initial investigation determined that the four-bladed propeller from the left inboard engine had separated in flight, subsequently impacting the left outboard engine, causing its propeller to separate also. Three years later, the left inboard propeller hub was recovered. All four blades had separated through the shank area adjacent to the hub. Detailed SEM examination confirmed a fatigue mode of failure in this area with a primary single origin on the inside surface of the shank. The main fatigue origin site was coincident with one of the defects on the inner surface of the blade shank. The most probable source for creating the defects on the ID bore of the shank was the blade tip chrome plating process, which was carried out during the last overhaul prior to the failure.