Taking aboard four coast watchers, "Grampus" sailed from Fremantle on 2 October 1942 for her fourth war patrol. Despite the presence of Japanese destroyers, she landed the coast watchers on Vella Lavella and Choiseul islands while conducting her patrol. This patrol, during the height of the Guadalcanal campaign, took "Grampus" into waters teeming with Japanese men-of-war. She sighted a total of four enemy cruisers and 79 destroyers in five different convoys. Although she conducted a series of aggressive attacks on the Japanese ships, receiving 104 depth charges for her work, "Grampus" was not credited with sinking any ships. On 18 October 1942 "Grampus" even scored a direct hit on the "Yura", but the torpedo failed to explode. She returned to Australia on 23 November.