return to England, Blackfish proceeded to the Royal Naval Dockyard at Devonport, England. She underwent repairs and testing there from 5 March to 4 April 1943. Fourth war patrol At 09:00 on 5 April 1943, Blackfish got underway for her fourth war patrol and headed for the waters of Inchmarnock at the northern end of the Sound of Bute in the Firth of Clyde on the coast of Scotland to perform practice approaches with the British patrol yacht . Gale-force winds, rain, and poor visibility hindered all training efforts, and by 15:00 Blackfish had shaped a new course, proceeding in company with the former French Navy minesweeper to Muckle Flugga in the Shetland Islands. On 7 April 1943 she parted company with La Capricieuse and made for the waters of Norway and Iceland. Overall, the patrol was uneventful with the exception of some environmental challenges. With air temperatures remaining between , a buildup of ice on Blackfish′s hull presented one of the more significant challenges of the voyage. In one instance the ice slowed her diving time by approximately two minutes while she tried to avoid an unidentified airplane. On 14 April 1943, Blackfish believed she sighted a periscope roughly off her port bow, but a subsequent search revealed nothing. It was the first of several sightings that likely were false as, according to Blackfish′s log, "a certain amount of wreckage has been sighted from time to time and a piece of this may have been mistaken for a periscope." The crew also spotted some mines and observed a few airplanes, most of which were friendly, but ultimately, encountered no enemy ships. Due to Blackfish′s position so far to the north and given the season, the consequent continual daylight made it possible Blackfish to conduct much of the patrol on the surface, and her crew believed that access to fresh air while surfaced made for better living conditions. In his reflections on the patrol Lieutenant Commaner Davidson observed that "the excessive smoking and steaming of the main engines in cold weather is a very serious military hazard and an immediate remedy is considered vital." On 11 May 1943, Blackfish anchored at Lerwick, Scotland, and at 10:00 on 12 May again got underway. In short order she joined Barb and La Capricieuse, and the three vessels headed for Rosneath in company. Blackfish anchored at Rosneath on 14 May 1943, concluding her