eight-ship convoy and underwent a seven-hour depth charging by surface ships and aircraft. From 15–17 June, heavy seas and high winds hampered hunting, and on 18 June, "a healthy stream of air bubbles" was discovered "issuing from the starboard side...." On 19 June, she left the area and headed for Balabac Strait. On 23 June, she moved into Makassar Strait, and on 4 July, she reached Fremantle. On her fourth war patrol, from 10 August-2 October, Seal returned to the Indochina coast and patrolled north from Cape Padaran. Despite 11 sightings, she was plagued by uncertain torpedo performance against shallow draft vessels, by premature explosions and by leaky exhaust valves and holes in the fuel compensating line which resulted in air and oil leaks to the surface. She was able to damage only one cargo ship, on 3 September. Twelve days later, Seal was en route back to Fremantle. She arrived on 2 October and departed again on 24 October to patrol in the shipping lanes in the Palau area. On 16 November, she intercepted a convoy of five cargomen in two columns with a destroyer escort and conducted a submerged attack on the leader of the near column as the formation zigzagged toward the submarine. Less than a minute after firing, Seal collided with, or was rammed by, another enemy ship. The periscope went black and vibrated severely. The submarine rose to ; hung there nearly a minute then started down. A few minutes later, depth charging began and Seal leveled off at . Breaking up noises were heard. Four hours later, the area was clear and Seal surfaced. The high periscope had been bent horizontally, and the housing on the low periscope had been sprung, preventing its operation. The radar antenna had been broken off the radio mast. Quantities of uncooked rice and beans, unlike those used on the submarine, were found between the wooden deck pieces of the cigarette deck, on the bridge, and caught in the bathythermograph. The periscope shears yielded "a good sample of Japanese bottom paint." Captured Japanese documents later confirmed the sinking of 3,500-ton Boston Maru by an American submarine on that date in that location. Whether that ship was Seals target or the colliding ship is not known, but it is possible that the freighter's hull had been badly punctured by the submarine's periscope shears. On 17 November, Seal was ordered to