The Malay Barrier was the site of "Grenadier"s fourth war patrol from 13 October to 10 December. After laying a minefield off Haiphong, Indochina, the submarine made an unsuccessful attack on a large freighter. During the severe depth charging which followed, sea water seeped into the batteries; "Grenadier"s crew suffered headaches and nausea from chlorine gas poisoning for the remainder of the patrol. On 12 November, she attacked and sank by torpedo the rescue tug () off the coast of Vietnam (11°18'N 109°02'E). To increase the misery, on 20 November "Grenadier" spotted a -class aircraft carrier, escorted by a cruiser and a destroyer, heading through the Strait of Makassar too distant to shoot. "Grenadier" surfaced to radio the aircraft carrier's location and course to Fremantle submarine base in hope that another submarine could capitalize on it.