The fatness of the 4th toe of the left foot may be caused by a variety of factors such as soft tissue injury, muscle ligament injury, frostbite, gout, and onychomycosis. 1. Soft tissue injury: wearing ill-fitting shoes for a long time will lead to local tissue damage when the toes are squeezed, resulting in swelling and pain in the 4th toe. 2. Muscle ligament injury: when the 4th toe is hit by external force suddenly, it will lead to the damage of local muscles, joint capsule and other soft tissues of the toe, which will produce inflammatory reaction and cause swelling. 3. Frostbite: If the fourth toe is stimulated by cold, it will lead to local blood circulation obstacles and bruised skin inflammation, resulting in swelling of the toe. 4. Gout: When the uric acid in the body is too high, it will lead to the deposition of uric acid crystals on the toes, which will cause the tissues of the toes to be infringed upon and lead to the symptoms of swelling and pain. 5. Inflammation of the nail: Inflammation of the nail in the 4th toe will cause localized edema, oozing and other reactions, resulting in swelling. If the 4th toe of the left foot is fat and painful, you should go to the orthopedic department of the hospital in time to find out the cause of the disease and treatment.