The 4th Fleet (第四艦隊, Dai-yon Kantai) was a fleet designation of the Imperial Japanese Navy. The Fourth Fleet designation was used during three separate periods. The initial designation was for a group of ships that were assigned to work together during the Russo-Japanese conflict and the period of its immediate aftermath. The second time the designation was used was during the Sino-Japanese conflict, and the third time was as a South Pacific area of command during the middle of the Pacific War.
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Russo-Japanese War
First established on June 14, 1905, the 4th Fleet was created after the Battle of Tsushima in the Russo-Japanese War specifically to support and cover the landings of Japanese forces in Sakhalin. Afterwards, it was sent to the United States with the Japanese delegation negotiating the Treaty of Portsmouth ending the war, and was disbanded on December 20, 1905.
The Fourth Fleet incident
While participating in war game maneuvers in 1935 the Fourth Fleet became caught in extremely foul weather. The weather continued to deteriorate and by 26 September had reached typhoon status. Two of the newer, large Special Type destroyers, the Hatsuyuki and the Yugiri, had their bows torn away by the heavy seas. A number of recently built heavy cruisers also suffered significant structural damage. The Myoko, the Mogami and the submarine tender Taigei developed serious cracks in their hulls, and the light aircraft-carriers Hōshō and Ryujo suffered damage to their flight decks and superstructure, with the Ryujo also having her hangar section flooded. The minelayer Itsukushima suffered damage that required several months for extensive repairs, necessitating a near complete rebuild. Nearly all the fleet's destroyers suffered damage to their superstructures, and fifty-four crewmen were lost, swept overboard or killed outright.
The Japanese Admiralty held a hearing on the damage suffered by Fourth Fleet in the storm, resulting in recommendations for changes on Japanese warship design and construction. A number of new designs that used heavier guns and taller superstructures were found to be top heavy, and efforts were made to stabilize these ships by reducing weight above the waterline. In addition, cracks in the hulls of the new cruisers indicated the recently adopted practice of electric welding hull seams was suspect, and the practice was canceled on all new Japanese warship construction. The event was kept a secret from the public.
Second Sino-Japanese War
On October 20, 1937, the 4th Fleet was resurrected as part of the emergency reinforcement program for the China Area Fleet after the North China Incident of 1937. The new 4th fleet was based out of Tsingtao and assigned to patrol the Bohai Sea and the East China Sea regions. However, unlike the IJN 5th Fleet, the 4th Fleet was never in actual combat. On November 15, 1939, the 4th Fleet was absorbed into the 3rd China Expeditionary Fleet under the overall aegis of the China Area Fleet. Although most of its ships were released for service with the Combined Fleet in the Pacific War a year later, most of the staff for the 4th Fleet remained in China, and were assigned to the Tsingtao Base Force for the duration of the war.
Pacific War
On the same date that the 4th Fleet was absorbed into the China Area Fleet, a new 4th Fleet was created to provide administrative control over Japanese naval forces in the Japanese-held island territories of the South Pacific (Caroline Islands, Marshall Islands, Mariana Islands, Palau). This third IJN 4th Fleet came under the aegis of the Combined Fleet on November 15, 1940. The operational name of this fleet was the South Seas Force. With the start of hostilities against the United States, the 4th Fleet was based out of Truk, with a secondary base at Kwajalein. After initial Japanese successes, additional bases were established in the southern Philippines, Guam, Wake Island, Gilbert Islands, eastern New Guinea, Bismarck Archipelago and the Solomon Islands.
After the Battle of the Coral Sea, the area covered by the 4th Fleet was reduced to an “inner core” of Japanese possessions, while the new IJN 8th Fleet was assigned to confront the advancing American forces in the Solomon Islands and New Guinea. However, in November 1943, the Americans attacked the Gilbert Islands and captured the major naval base of Tarawa, which brought the 4th Fleet and its various garrison forces back into the front lines of combat.
The Americans continued to advance through the Marshall Islands in early 1944, neutralizing the Japanese bastion at Truk Atoll in the Carolines, headquarters of the IJN 4th Fleet, by means of powerful naval air strikes in February, and forcing the removal of Japan's major naval units to Palau, which also proved vulnerable to air attack.
In March 1944, the IJN 4th Fleet came under operational control of the Central Pacific Area Fleet based in Saipan. It effectively ceased to exist with the fall of Saipan to American forces.