In the Line of Duty 4: Witness (a.k.a. In the Line of Duty Part 4) is a 1989 Hong Kong action film directed by Yuen Woo-ping. The film stars Cynthia Khan, Michael Wong, and Donnie Yen. The film was released theatrically in Hong Kong on 21 July 1989.

The film is nominally part of the loosely connected In the Line of Duty and Yes, Madam! series. The alternative titles of this and other films in the series can be cause for confusion.

Hong Kong international police officer Rachel Yeung Lai-ching went to the United States to investigate a drug trafficking case in Seattle with the local narcotics team leader, Tony. When they tracked down a local Chinese suspect, Liu Zai, they had already received news that Liu Zai had sneaked back to Hong Kong, so Yeung Lai-ching and Tony returned to Hong Kong together. During the pursuit, they found that Liu Zai was constantly being chased and almost died several times. Tony began to think that Liu Zai was definitely not just involved in this drug trafficking case. When Tony and Yeung Lai-ching continued to investigate, they found that the whole thing was actually related to the US Central Intelligence Agency 6, and Tony's colleague, the leader of another group, Michael, was also involved. The truth of the incident was really revealed layer by layer.

• Cynthia Khan as Madam Rachel Yeung Lai-ching
• Michael Wong as Captain Michael Wong
• Donnie Yen as Captain Donnie Yan
• Yuen Yat-chor as Luk Wan-ting
• Lisa Chiao as Luk's mother
• Liu Kai-chi as Ming
• Paul Wong as Hospital Killer
• Fairlie Ruth Kordick as Blonde fighter
• Jim James as Officer John

TV Guide said: "Although the action comes neatly packed, screenwriters Anthony Wong and Chueng Chi Shing fill the story with so many double crosses that it sometimes unravels under their weight."^[1]

• Donnie Yen filmography
• List of Hong Kong films of 1989

• In the Line of Duty 4 at IMDb
• In the Line of Duty 4 at HKMDB
• In the Line of Duty 4 at Hong Kong Cinemagic (archived)