Lorna Cole (Rene Russo) is pregnant with LAPD Sergeant Martin Riggs' baby; they are not married, but both are thinking about it. LAPD Sergeant Roger Murtaugh's daughter Rianne (Tracie Wolfe), is also pregnant, though he is not told the child is with Detective Lee Butters (Chris Rock) since he did not want his daughter to marry a police officer. Due to issues with the department's insurance carrier over Riggs and Murtaugh's actions as sergeants, Captain Murphy (Steve Kahan) promotes them to captains.
The officers along with Leo Getz (Joe Pesci) come upon a Chinese immigrant smuggling ring after running an ocean-going vessel aground. The captain of the boat escapes. In the subsequent investigation, Murtaugh finds Hong (Eddy Ko) and his family hiding from US Immigration officers to avoid deportation. Murtaugh offers them shelter in his home, and their families, along with Riggs and Cole, quickly bond. Hong reveals he is looking for his uncle, a skilled engraver, who had paid for their passage to the United States.
Continued investigation of the smuggling ring leads the officers to "Uncle" Benny Chan, a crime boss operating from a Chinatown restaurant. There, they are introduced to high-ranking Triad negotiator Wah Sing Ku (Jet Li). Chan forces them out of his restaurant when the officers show no probable cause. Outside, Riggs spots the escaped captain and tries to give chase but fails to capture him. However, Ku later intercepts the captain and kills him as punishment for attracting the police's attention. Chan and Ku take steps to eliminate all those in on their plans.
Hong is able to contact his uncle, but this leads the Triads to Murtaugh's house. Ku and the Triad thugs kidnap the Hongs, tie up Murtaugh, his family, Riggs and Cole and set the house ablaze. Ping, Hong's son, has evaded capture and helps to free the others in time. Though Riggs and Murtaugh chase down and kill some of the Triad, Ku escapes. Ku brings Hong to his uncle at a warehouse, where he has been engraving plates for a counterfeiting operation for Ku and Chan in exchange for transit for his family. Ku kills Hong in front of his uncle to assure his continued work.
The officers attempt to shake Chan for more information, but are unable to get any leads until they discuss what they know with Detective Ng (Calvin Jung), who has worked on cases involving the Chinese government before. Ng recognizes that Ku must be trying to negotiate with a corrupt Chinese general for the release of the Four Fathers, high-ranking Triad members that include Ku's brother (Conan Lee). New information leads them to the abandoned warehouse where they find the bodies of Hong, his uncle, and Chan, the latter two killed by Ku after they served their usefulness. Knowing that Ku plans to use counterfeit money, the police intercept the exchange between Ku and the general, telling the general the money is fake. A firefight breaks out between the Triad, the general's private army and the police, and most of the Triad and army are killed. Ku attempts to escape with his brother, but his brother is shot and killed by Murtaugh. Riggs and Murtaugh are separated as they follow Ku to a pier. Riggs engages in a fistfight with Ku that collapses the pier and sends them underwater. Riggs is able to find a gun and kill Ku, while Murtaugh arrives in time to help rescue Riggs from a piece of concrete that had pinned him underwater.
Later, Riggs visits his dead wife's grave and asks her for advice about his impending marriage with Lorna, about which he still has doubts; Getz arrives and offers a childhood story that gives Riggs a new light on the situation. They learn Lorna is about to give birth and race to the hospital, where Riggs and Lorna are ceremonially married by a rabbi just before she enters labor. Their son and Rianne's daughter are born, and Murtaugh accepts Butters as his son-in-law. Murphy gives Riggs and Murtaugh their rank of Sergeant back since the city is insured again and Hong's family is granted asylum.Mel Gibson as Martin Riggs
Danny Glover as Roger Murtaugh
Joe Pesci as Leo Getz
Rene Russo as Lorna Cole
Chris Rock as Detective Lee Butters
Jet Li as Wah Sing Ku
Kim Chan as Uncle Benny Chan
Steve Kahan as Captain Ed Murphy
Calvin Jung as Detective Ng
Jack Kehler as U.S. State Department Official
Eddy Ko as Hong, Chinese Refugee
Mary Ellen Trainor as Dr. Stephanie Woods
Conan Lee as Ku's brother
Darlene Love as Trish Murtaugh
Traci Wolfe as Rianne Murtaugh-Butters
Damon Hines as Nick Murtaugh
Ebonie Smith as Carrie Murtaugh
Writer Jeffrey Boam who worked on some uncredited re-writes of the first film, completely re-wrote Shane Black and Warren Murphy's script for second film and wrote the script for third film originally wrote a draft for fourth Lethal Weapon film in January 1995. This draft had Riggs and Murtaugh dealing with racist white trash right wing neo-nazi survivalist militia group committing a terrorist attack in L.A. Boam said how the story of the script dealt with real life neo-nazi activity in US and how it was written to be more serious sequel focusing on the story and less on episodic comedy parts like the third film did. He also said how his script was lot more exciting and how the plot of the script that was eventually used for the filming which dealt with Chinese money counterfeiters and immigrant smugglers was very low stakes type of plot and not suspenseful enough for a Lethal Weapon film.
After Boam's draft was rejected for unknown reasons, although producers and studios decision to keep the focus of the next sequel on comedy was mentioned to be the possible reason for it, several other ideas and drafts were suggested for the film. Producer Joel Silver eventually hired several writers, including TV writer Channing Gibson who had never worked on a theatrical movie and who received credit for the story, to write a draft which included Chinese immigrants but it turned out to be a problem once the script had to be written and re-written again and again. Even though four writers are credited for the script (Channing Gibson, Jonathan Lemkin, Alfred Gough and Miler Millar), a lot more people were involved in re-writes during production. By the time filming began not even half of the script was finished and the ending was constantly being re-written until it was time to actually film it. The production problems and delays caused due to the issues with constant re-writes left only one month for the movie to be finished for theatrical release. In the very first script for the film Leo Getz and detective Lee Butters were not included, and once Joe Pesci and Chris Rock were brought in at the last minute even more re-writes were required. Butters was first written to be a gay character but cast and crew felt that with the way he was written it didn't work so he was changed to be Rianne's husband.
Lethal Weapon 4 debuted at No. 1 at the box office. Although the film grossed $130 million in the U.S., it was not considered a runaway financial success as the previous three films had been. Shooting began in January 1998, just months before the film's release, with a production budget estimated at $120–$150 million (although Warner Bros. maintained it cost less than $100 million) and an additional $50 million spent on marketing and distribution. This made the fourth film the most expensive entry of the series. Its profit margin was saved in part due to the combined foreign box office sales making the film gross approximately $285 million in total. Still, like its predecessors, Lethal Weapon 4 was among the top ten grossing films of its release year.
Critical reaction to Lethal Weapon 4 was mixed. The film currently holds a rating of 52% on Rotten Tomatoes based on reviews from 66 critics, and an average rating of 5.8 out of 10. Metacritic, which assigns a normalized rating of 0-100 from mainstream critics, calculated a score of 37 out of 100 based on reviews from 21 critics, which means it has an indication of "generally unfavorable reviews". James Berardinelli gave the film three stars out of four, writing: "Given the expectations that constrain it, Lethal Weapon 4 is probably the best motion picture that could possibly result from another teaming of Martin Riggs and Roger Murtaugh. The series has lost a lot of steam since the first two entries, and, although the fourth movie ratchets up the energy level from the moribund state of the disappointing Lethal Weapon 3, there's no sense of spontaneity." Roger Ebert gave Lethal Weapon 4 two stars out of four, writing: "Lethal Weapon 4 has all the technical skill of the first three movies in the series, but lacks the secret weapon, which was conviction. All four movies take two cop buddies and put them into spectacular and absurd action sequences, but the first three at least went through the motions of taking the plot seriously (and the first one did such a good job, it made my 'best 10' list of that year). This time, we're watching an exercise." Michael O'Sullivan of The Washington Post called it a "stupid and violent delicacy" that balances a "patented blend of high action and low comedy".
The film was nominated for a Razzie Award for Worst Supporting Actor for Pesci.
Lethal Weapon 4 has been released on VHS and DVD numerous times. It has been re-released in numerous sets that contain all four films in the series. Lethal Weapon 4 was released on Blu-ray Disc as part of a box set with the additional Lethal Weapon films on May 22, 2012.
The film's music was composed by Michael Kamen, Eric Clapton, and David Sanborn. This was the only film in the series not to have a soundtrack album released alongside it, but in 2013 La-La Land Records released the score as discs seven and eight of its limited edition Lethal Weapon Soundtrack Collection.