On 8 July 1942, I-171 departed Kwajalein on her fourth war patrol, tasked to conduct a reconnaissance of Fiji and the Samoan Islands during the patrol. Between 16 and 24 July 1942 she operated in the Fiji area, then, after reporting no ships in the harbor at Suva, proceeded to Samoa, where she reconnoitered Pago-Pago on 28 July 1942. On 29 July 1942, she fired a single torpedo at an unidentified merchant ship off Tutuila, but missed. She concluded her patrol by arriving at Truk on 12 August 1942.
