Yu-4 (鱼-4) torpedo is the Chinese development of the Russian SAET-50 ASuW passive acoustic homing torpedo, and it is the Russian SAET-50 torpedo incorporating active acoustic homing guidance. Yu-4 torpedo is often erroneously claimed by many sources as the Chinese version of the SAET-60 acoustic homing torpedo, but Soviets had not completed the development of SAET-60 and only fielded the weapon in 1961, by then the relationship between former-Soviet Union and China had already worsened and China had not received any technical assistance on SAET-60 torpedo. The background of the development of Yu-4 torpedo is identical to that of Yu-1 torpedo and Yu-2 torpedo.