Jeffrey Boam, who worked on uncredited re-writes for Lethal Weapon (1987) where he added some more humor into the script after Richard Donner thought that original script was too dark, did a complete re-write of the script for Lethal Weapon 2 (1989) after original draft by Shane Black and Warren Murphy was rejected for being too dark and bloody and because Riggs died at the end, and wrote the script for Lethal Weapon 3 (1992), wrote a completely original script for fourth Lethal Weapon movie which everyone at Warner Bros. had liked, but was not used. Boam's unused script was built around Riggs and Murtaugh dealing with racist white trash right wing neo-nazi survivalists militia group who were doing a terrorist attack in L.A. This script was written sometime in January of 1995 and according to Boam it dealt with entire real life neo-nazi activity in US that was only superficially dealt with only one movie before, Dead Bang (1989). Interviewed after the release of Lethal Weapon 4, he remarked that he thought the counterfeiting of Chinese money was hardly a matter to create the suspense appropriate to a Lethal Weapon film and also said how his script was much better from the final movie which suffered some problems with constant re-writes of the script that was chosen for the filming and actually didn't had completed script during filming.
When Riggs and Lorna were having the discussion about her hearing a rumor that Murtaugh was "on the take." Riggs jokingly said "I've tried to convince him to take money." In Lethal Weapon 2 (1989) when they were both trapped in a storage container filled with the drug dealers' money, Riggs did try to convince him to take some of the money and do something good with it.
Paul Tuerpe appears in all four Lethal Weapon movies, but always in slightly different roles. He played 'Mercenary' in the first, 'Hitman' in the second, 'Henchman #3' in the third, and 'Helicoptor Co-pilot' in the fourth.
The car chase and fight between Riggs and the Chinese man in the house trailer was filmed on the 215 freeway in Las Vegas. Apparently, they couldn't get filming permission anywhere, but the Las Vegas mayor was very accommodating. All drivers were either stunt drivers or members of Bill Young's Driving Team.
Started filming in January, seven months before its release date. The shoot ended exactly 33 days before the release date. According to editor Frank J. Urioste, to meet the deadlines he had to use Avid, even though he had never edited a whole movie using digital technology before.
Roger Murtaugh's (Danny Glover) line of "I'm too old for this shit!" has made its way into every Lethal Weapon movie. Only this time, Riggs amends it to "We're not too old for this shit!" as a sign of defying their ages.
After Joe Pesci and Chris Rock were hired at last minute to play characters of Leo Getz and Lee Butters, characters who weren't in original script, some re-writes had to be done during filming to include both of them into the story. Butters' character was originally going to be a gay character but once filming started, everyone involved realized how the decision to make him gay didn't work, so his character was re-written again to be a husband of Roger's daughter Rianne.
Only 'Lethal Weapon' film not to feature any scenes in Murtaugh's bathroom. The first one had the birthday cake scene in the beginning. The second one had the toilet bomb scene. The third one had the retirement cake scene toward the end.
Warner Brothers decided during the scripting process that the only story line they wanted to do involved Chinese immigrants in some way. Several writers worked out of this basic outline, with Channing Gibson earning the assignment; the story elements involving the Triads and corrupt officials were added in once the film was green-lit.
The film's screenplay was in flux all the way from development through the entire production schedule. Channing Gibson, who took on primary scripting duties due to his longstanding work history with producer Joel Silver, was a successful TV writer who was looking forward to the more relaxed pace of screenwriting, ended up going through more revisions than any TV show he'd worked on. Amongst other things, the character of Lee Butters was not in the original script, and hastily enhanced when Chris Rock signed on, and Joe Pesci's Leo Getz wasn't slated to be part of the film until Richard Donner and Silver recruited Pesci to reprise his role.
In one scene, Riggs and Murtaugh congratulate one another on their promotions by rapidly alternating between handshakes and salutes. This is similar to scenes in Forever Young (1992) between Mel Gibson and George Wendt.
Promotional television ads and the theatrical trailer featured Chris Rock wearing a police uniform doing a comedy act (he is referring to himself as Riggs being cast with a different actor - himself). His character never wore a police uniform in the movie. During the photo album credits, a picture of him wearing the uniform can also be seen.
The second Lethal Weapon movie to feature the casting credits at the end of the movie, as opposed to the beginning. The first of the series to do this was Lethal Weapon 2 (1989). Both Lethal Weapon (1987) and Lethal Weapon 3 (1992) had their casting credits at the beginning of the movie.
One of the Pontiac Grand Ams used in the film was on display at Warner Brothers Movie World, Gold Coast, Australia, for several years, before being auctioned off along with other items from the park in late 2012.
On the Beretta 92, a button on the right side of the frame must be pushed in to un-block the lever on the left side, which is rotated down 90 degrees, in order to release the slide so the weapon can be 'stripped' and cleaned. It is possible to grab the gun, push the tab down, and pull the slide off to disarm the weapon. When Wah Sing Ku pulls the slide off of Riggs' Beretta 92, the button has already been pushed down and the lever partly rotated.
Only movie of the 'Lethal Weapon' series that does not feature the chase theme. The chase theme was used in the first one during the Hollywood Boulevard chase scene when Riggs pursues Mr. Joshua. It was used in the second one during the pursuit of the stolen tow truck. It was used in the third one during the pursuit of Jack Travis at the hockey rink.
Only film in the 'Lethal Weapon' series in which Riggs does not drive a GMC Sierra with dual rear wheels. In this film, Riggs drives a GMC Sierra with single rear wheels. It is also the only film in the series in which Riggs drives a GMC truck with 4-wheel drive (the trucks in the first 3 films were only 2-wheel drive).
After being promoted, Riggs addresses Murtaugh with the words "O Captain my Captain", which is the title of a poem by Walt Whitman (and, of course, a reference to the Dead Poets Society (1989) where the phrase plays a central role.)
In a scene in the police station the viewer can see posters advocating gun control. This drew some criticism in certain circles and even some unintentional humor due to the earlier films near glorification of weaponry and the sometimes over the top use of said weaponry by the police officers in the films.
The second locomotive that strikes the bad guys' car in the second railroad crossing scene is the same former Alaskan Railroad GP7-type locomotive No.1804 which was at the front of the Grand Continental train in Under Siege 2: Dark Territory (1995). It has the same livery, with the Grand Continental names removed.
The end hospital scene has two characters from Fletch (1985). Richard Libertini is Fletch's boss and here he is a Rabbi. Bill Henderson is the speaker at Fred Dorfmans party and here is the Angry Patient with Urine Sample.
The trivia items below may give away important plot points.
Originally, Mel Gibson insisted his character be killed off in the movie as he felt this should be the last Lethal Weapon movie. After filming was completed he retracted his statements, saying he had such fun making the movie he'd be happy to do another one. The film teased fans with his original statements by hinting he was going to die by drowning after the concrete slab fell on him while Murtaugh was still unconscious.