西里伯斯海
"Rasher" returned to Makassar Strait-Celebes Sea area for her fourth patrol, from 30 April to 23 June. On 11 May, she torpedoed and sank the freighter "Choi Maru". Next to go down were the converted gunboat "Anshu Maru" on 29 May and the tanker "Shioya Maru" in the Celebes Sea off Manado 8 June. Six days later, the cargo ship "Koan Maru" went to the bottom, after taking a spread of torpedoes aft and capsizing.