The movie's title, "Live Free or Die Hard", is a reference to New Hampshire's State Motto "Live Free or Die". Consequently, the New Hampshire state film office received several phone calls asking where in the state the movie was filmed.
It took four months to assemble and combine archive footage of past American presidents from Franklin D. Roosevelt to George W. Bush to create the televised warning from Gabriel. The goal was to create a video representation of a ransom note.
In the elevator shaft scene where Mai Lihn swings on the wire and flies into the truck and hits McClane, the stunt double accidentally cut Bruce Willis' eyebrow with her spiked heel and according to Len Wiseman in the DVD Commentary, she jabbed Willis hard enough that when medics examined the injury, the brow bone was exposed.
Kevin Smith rewrote the lines for his Warlock character. Bruce Willis thought the rewrite was too funny and did not follow the serious mood of the movie. Smith then rewrote the part to what Willis requested.
This film addresses the apparent continuity error in earlier installments - McClane is afraid of flying in Die Hard (1988) and Die Hard 2 (1990), but not Die Hard: With a Vengeance (1995). Here, he explains that he took flying lessons in order to "face his fears."
Bruce Willis' stunt double, Larry Rippenkroeger, was seriously injured when he fell 25 feet to the pavement. He suffered broken bones in his face and fractures in both wrists. Production was temporarily shut down. Willis picked up the tab at area hotels for Larry's parents and visited him a number of times at the hospital. Larry also doubles for James Caan in his TV series, Las Vegas (2003). Caan came and visited Larry in the hospital and joked around for over an hour. Larry told his parents he was glad when Caan left because he hurt so bad laughing at Caan's jokes.
The stunt featuring McClane driving a police car into a helicopter took three weeks to rehearse. The shot was accomplished by suspending the helicopter in the air with cables and combining two separate shots; one of the stuntman leaping from the helicopter and one of the car colliding with it. Computer animation was then used to delete the support cables and add rotor blades.
When McClane is driving toward the helicopter, he says, "You think a traffic jam, throwing a car at me is gonna stop me, huh?" Director Len Wiseman dubbed the last part of this line ("Gonna stop me"), imitating the voice of Bruce Willis.
The bad guy Thomas Gabriel points a gun at McClane and declares "On your tombstone it will say 'Always in the wrong place at the wrong time'." "John McClane is back in the wrong place at the wrong time!" was a tagline used for Die Hard 2 (1990).
According to Bruce Willis and Director Len Wiseman in the DVD Commentary, the story originally involved McClane's son, Jack. Originally, he was supposed to be the computer hacker John has to deliver to the FBI. Eventually that idea was dropped and the hacker became the Matt Farrell character. It was then decided to bring in his daughter Lucy to keep up the series theme of McClane always having a personal stake in what happens in the story.
The first sequel in the series where no character mentions John McClane's encounter at the Nakatomi Plaza in L.A. in Die Hard (1988). However, visual references to Nakatomi Plaza are seen while Thomas Gabriel is looking at John McClane's dossier.
In the beginning credits when Kevin Smith's name comes on the screen. The "m" in smith disappears and you see "Sith" for a few seconds paying homage to Kevin Smith's love of all things Star Wars which also reflects in his character in the movie.
The first film in the series where the antagonist does not use a fake accent. In Die Hard (1988), Hans Gruber faked an American accent to fool McClane; in Die Hard 2 (1990), Colonel Stuart fakes a Midwestern accent while guiding an airline pilot into crashing into the ground; and in Die Hard: With a Vengeance (1995), Simon fakes an American accent while posing as a city engineer.
In addition to the 'Agent Johnson' reference, several other elements from the first Die Hard (1988) film are revisited as series trademarks. Among them are: crawling on broken glass, use of air-ducts, elevator shafts, and maintenance areas in corporate buildings, a henchman falling down stairs, an inquiry on the E.T.A. of a helicopter, and McClane's "Yippie Ki Yay' catchphrase.
A water treatment facility near Los Angeles doubled as the film's Woodlawn Social Security Administration building. The facility has miles of underground tunnels, and was also used in Die Hard 2 (1990) when Bruce Willis runs through tunnels under the airport.
When filming the scenes of John walking through the corridors talking to Gabriel on the two-way, there were no written lines of dialog for Bruce Willis, according to Len Wiseman on the DVD Commentary. So what they did on set was have Willis hold the two-way up to his mouth and speak gibberish so it looks like he's talking to Gabriel. If you'll notice, there are a couple of times where the two-way isn't all the way up to Willis's face and you can see his mouth doesn't match the dialog being spoken.
The name "Tovarek", which Mai Lihn uses as an FBI agent, is a Polish word and one of its meanings is "hot chick" (the correct Polish word is "towarek", but it's pronounced like this). Tara Tovarek is also the name of producer Michael Fottrell's assistant.
The film was edited down to a PG-13 rating for commercial reasons, thus making it the first film in the series not rated R. The DVD version is unrated and restores much of the profanity and violence that was trimmed for this purpose, making the film an equivalent of an R rating.
The only Die Hard film that is set in multiple locations (New Jersey, Washington DC, West Virginia, and Baltimore). The other three Die Hard films were all set in and around (or above) primarily one location throughout most of the movies.
When Gabriel is talking to McClane over the phone and pulling up his information on the computer, Bonnie Bedelia makes a cameo appearance in the form of her character Holly McClane's driver's license photo. The photo appears as though it may be either a publicity shot from a prior "Die Hard" film or a still photo (i.e. family portrait) from one of the movies.
This is the first Die Hard film without the music of film composer Michael Kamen. Kamen died in 2003. Portions of Kamen's previous "Die Hard" scores, however, were incorporated into the score by Marco Beltrami.
In the beginning of the film, John McClane and Lucy have an argument that eventually leads to them discussing her use of her mother's last name, "Gennero". A similar argument takes place in Die Hard (1988), when McClane is searching for the location of his wife, Holly, in the Nakatomi building. He does not find her under "McClane," but does find her under her maiden name "Gennaro", which is misspelled.
On the DVD commentary, director Len Wiseman discusses the "back yard Die Hard (1988)" he shot when he was younger. At least one of the shots from it was used in this film, and Wiseman acted it out for Bruce Willis during filming so that Willis could duplicate it.
When McClane is driving in Jersey and talking to the chief captain of Camden, the guy's name is Wiseman, same name as the director, Len Wiseman, although pronounced 'Wheezeman', as this was how most crew-members thought his name should be pronounced. The voice is that of the director himself.
All the IP-addresses shown in the movie are legal ones. However, most start with either 10, 172.16 or 192.168. Those numbers are reserved for local traffic only. At 02:04, one of the hackers transfers data with scp to 18.104.22.168, which is owned by a Japanese company.
The car that is stolen in the film by McClane and Farrell is a 2006 E60 BMW 5 series, which was chosen due to a poll that found that people wanted films that had more BMWs in it. The main reason given was that the alternatives (Audis and Mercedes-Benzes) were too common and not bold and imposing enough to go with the characters in the film. The particular BMW model (5 series) was chosen because the director, Len Wiseman, found "the 3 series too common, the 7 series too uptight and every other car either too feminine or compensating for a midlife crisis... Everything McClane isn't, yet".
While in early development with a script that was eventually discarded, the movie had been given the subtitle "Tears of the Sun". Bruce Willis told the studio he would commit to a Die Hard 4 if he could use the title for Tears of the Sun (2003).
Prints were sent out to UK cinemas under the fake name "New Hampshire" - a reference to the state's "Live Free or Die" motto and the movie's original title - in spite of the title being changed to "Die Hard 4.0" in European territories.
The screen to the far left of all of Warlock's hacking screens has the auction website eBay open with a Boba Fett action figure being watched, despite the fact that the power had already been cut off by that time in the screenplay.
According to the files that Thomas Gabriel pull up on Holly Gennero, her Driver License records list her address as the fictitious 9975 Geyser Way East, San Francisco, CA, 9424- (the smaller picture on her license obscures the final ZIP code digit).
The trivia items below may give away important plot points.
Director Len Wiseman credits actor Timothy Olyphant (Thomas Gabriel) with coming up with the idea of how McClane kills Gabriel. In order to keep it a secret, the scene was not included in the film's shooting script.
The film's climax originally involved McClane racing alongside an exploding gas pipeline on a motorcycle with Matt in a sidecar. The idea evolved into Gabriel trying to kill McClane and Matt by routing gas to their location through the pipelines.