OVERVIEW: The Yu-4B is a Chinese submarine-launched heavyweight torpedo with active/passive acoustic homing, developed for anti-surface warfare. It entered service in the late 1980s as an improved successor to the passive-only Yu-4A.

DETAILS: "Yu" (鱼) means "fish" in Chinese torpedo classification, with "4" indicating its place in the series, and "B" designating the upgraded active/passive homing variant. The Yu-4B was developed from the Soviet SAET-50 and evolved from the Yu-4A by integrating active sonar guidance to enhance effectiveness against maneuvering targets. It is powered by an electric propulsion system using silver-zinc batteries and features a 309 kg high-explosive warhead. The Yu-4B offers improved performance in range, speed, and target acquisition compared to its predecessor.

TYPE: Heavyweight Anti-Surface Warfare Torpedo

FUNCTION: After submarine launch, the Yu-4B uses wire-independent mid-course navigation followed by terminal active/passive acoustic homing to engage surface ships. Its onboard sonar allows it to detect, track, and strike moving surface targets with greater accuracy than earlier passive-only torpedoes.

NOTE:
IOC: 1984 (Yu-4A), 1989 (Yu-4B)
Manufacturer: China (CSIC / associated defense factories)
Operators: China
Platforms: Type 035 Ming-class, Type 039 Song-class submarines

SOURCE:
Wikipedia: https://en.wikipedia.org/wiki/Yu-4_torpedo ; GlobalSecurity.org: https://www.globalsecurity.org/military/world/china/yu-4.htm ; Jane’s Naval Weapon Systems ; Friedman, The Naval Institute Guide to World Naval Weapon Systems