On 23 February 1943, "Wahoo" got underway for Midway Island, where she arrived four days later, topped off her fuel tanks, and headed for her patrol area. For "Wahoo"s fourth patrol, Morton was assigned to the extreme northern reaches of the Yellow Sea, in the vicinity of the Yalu River and Dairen, an area never before patrolled by U.S. submarines. One reason for this was the water was shallow, averaging . While "en route" to her patrol area, she conducted training dives, fire control drills, and battle surface drills. She had the unique experience of making the entire passage to the East China Sea without sighting a single aircraft, thus making most of the trip surfaced. On 11 March, "Wahoo" arrived in her assigned area along the Nagasaki-Formosa and Shimonoseki-Formosa shipping routes.