Fourth patrol, February ? April 1943
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 United States submarines. One reason for this was the water was extremely shallow, averaging 120 feet (37 m). 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 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.
On 19 March 1943, the shooting began with a freighter identified as Zogen Maru. A single torpedo hit broke the target in two; the aft end sank immediately, and the bow sank two minutes later. There were no survivors. Four hours later, Wahoo sighted another freighter, Kowa Maru, and launched two torpedoes. The first hit under the target's foremast with a terrific blast, leaving a tremendous hole in her side, but the bow remained intact. The second torpedo hit amidships, but it was a dud and did not explode. Two more torpedoes were fired, but the freighter maneuvered to avoid.
A Japanese freighter Nittsu Maru sinks by the bow after being torpedoed by WahooWahoo then patrolled off the Korean coast, just south of Chinnampo. On 21 March, she sighted a large freighter identified as Hozen Maru. She launched three torpedoes; the third hit the target amidships. She went down by the bow, sinking in four minutes, leaving approximately 33 survivors clinging to the debris.
Four hours later, Wahoo sighted the freighter Nittsu Maru. The submarine fired a spread of three torpedoes; two hit, one under the bridge and the other under the mainmast. The ship went down in three minutes. Four survivors ignored all efforts to rescue them. After collecting a few souvenirs from the scattered wreckage, Wahoo commenced a surfaced patrol, heading for Shantung Promontory. On 22 March, the submarine headed for Laotiehshan Promontory, close by Port Arthur.
The following day, as Wahoo patrolled Laotiehshan Channel (also known as "Sampan Alley"), she found herself surrounded by targets. Wahoo sighted a medium-size ship, apparently a freighter, the collier Katyosan Maru, and launched one torpedo. This hit just under the bridge, immediately enveloping the target in a screen of coal dust. The maru settled fast and slowed down, vanishing in 13 minutes.
Wahoo set course for a point to the northeast of Round Island, off Dairen. In the vicinity of the port's approaches, the deepest water is about 300 feet (91 m), with an average depth of only 120 feet (37 m).
On 24 March, at 12:47, Wahoo sighted smoke and began to make her approach. At 19:49, she fired a spread of three torpedoes at a large tanker (identified as Takaosan Maru) which was fully loaded with fuel oil. The first two torpedoes exploded prematurely; the third missed. Wahoo fired a fourth shot which also missed. The target commenced firing deck guns at the American warship. The submarine surfaced after 14 minutes of ducking shots, gained position ahead, and dove. She fired another three-torpedo spread. One hit the engine room and sank the ship in four minutes.
The next day, Wahoo sighted freighter Satsuki Maru. She launched two torpedoes; when both exploded prematurely, Wahoo battle surfaced to use her guns. She closed the target, raked her with 20 millimeter shells and holed her with almost 90 rounds of four-inch (102 mm) . The target caught fire in several places and sank in about one hour.
Wahoo left the following morning to investigate a ship on the horizon; the target proved to be a small diesel-driven freighter. The submarine commenced firing with her 20 millimeter and four-inch (102 mm) guns. The freighter tried to ram, but Wahoo kept clear, and continued firing at the target, setting it ablaze from stem to stern and leaving her dead in the water. The crew alternated looks through the periscope as the freighter sank.
Later that day, Wahoo sighted a 100 ton trawler and again attacked with her deck guns. When all three 20 millimeter guns jammed, Wahoo went alongside the riddled trawler and the Wahoo sailors hurled homemade Molotov cocktails (gifts from the Marines at Midway) onto the trawler. Wahoo departed, leaving the ship wrecked, spouting flame and smoke. On 28 March, while on the surface astride the Shimonoseki-Formosa shipping route, Wahoo opened fire with two 20 millimeter guns on two motor sampans. The targets did not sink but were also left wrecked.
The following day, Wahoo sighted the freighter Yamabato Maru and fired two stern tubes. The first torpedo hit at the point of aim under the mainmast and completely disintegrated everything abaft the stack. The forward section sank in two minutes. The second torpedo was aimed at the foremast; it missed because the first torpedo stopped the freighter in its tracks.
Wahoo surfaced, transited Collnett Strait, and headed home, concluding a war patrol which topped the record to date in number of ships sunk. Pearl Harbor reported that "Japanese think a submarine wolf pack operating in Yellow Sea. All shipping tied up."
Meanwhile, the United States mounted its offensive against Attu, and Admiral Koga Mineichi returned his major units from Truk to Tokyo Bay for the sortie to Alaska. Forewarned by codebreakers the Japanese intended to counter the Attu invasion by a major sortie of the fleet, COMSUBPAC sent his top sub, Wahoo, to the Kuril Islands to intercept it.
On 6 April 1943, Wahoo arrived at Midway, and commenced refit the following day. On 21 ? 22 April, she conducted training underway and was declared ready for sea on 25 April.