The seamless transitions between separate scenes in the film were not created using (digital) visual effects, but were done practically. For these transitions, the sets were build in such a way that two separate scenes could be filmed in one shot without interruption.
Darren Lynn Bousman was originally not interested in directing the fourth installment in the "Saw" series, but the producers persuaded him to read the script anyway. Bousman decided to take the directional duties when a plot twist in the script took him completely by surprise, something he thought impossible after being involved in the "Saw" series for three years.
Though producers wanted Donnie Wahlberg to reprise his role, his schedule made him unavailable and his character was not included in the original script. However, after shooting began, his schedule freed up, and the script was re-written to include his character.
As part of a running gag in the "Saw" series, the filming crew always hides what they call a "fart machine" in one of the sets, and make it go off in a particular serious scene, to see how long the actor or actors can keep a straight face. This time, Lyriq Bent was the victim. He actually managed to remain in a character for a couple of seconds before bursting into laughter.
Saw IV (2007) was given the green light before the third film even opened. This was stated in Fangoria Magazine #257: "The synopsis, bare as it is, takes the third installment to a new budget level for Lionsgate's wildly successful franchise. Saw (2004) was made for approximately $1 million, Saw II (2005) cost $4 million and the figure for Saw III (2006), according to producer Mark Burg, has risen to $10 million. Saw III was green-lighted the morning after Saw II opened to massive box-office returns, the producer confirms, 'and when they ordered Saw III, they also ordered and green-lit Saw IV.'"
The parade that Jigsaw and Cecil are at is celebrating the Year of the Pig in the Chinese Zodiac. The pig is the same animal in which Jigsaw uses to mask himself when capturing victims. Additionally, the release year of this film (2007) is actually the Year of the Pig, which occurs every twelve years.
The black figure Cecil steals from the Year of the Pig festival is the same figure John was seen placing on a shelf in the operating room in Saw III (2006). There is a deleted scene that shows John placing the same figure on Gideon's tombstone but this scene never made it into the final film.
This is the first "Saw" film written by Patrick Melton and Marcus Dunstan, who would also write the next three entries in the series. When interviewed about the job, they learned that there was a "Saw Bible" with rules they should have followed; however, as they had never received it, they had planned out an entire series of "Saw" sequels from their own ideas. Fortunately, their pitch was well-received by the studio and they got the job.
When the results from the fingerprint analysis from the bullet come in, two other names can be seen on the sheet that matches the fingerprint on the bullet to Officer Rigg: "Entry Employee: Scoffield, Sean" and "CSI Case App: Armstrong, David". Sean Scoffield is in charge of doing the graphics for "Saw IV" and David A. Armstrong is the Director of Photography.
The character of Officer Rigg (Lyriq Bent) was promoted from a supporting character in Saw II (2005) and Saw III (2006) to a main character in this one. Bent later said that there may have been plans to make him a central character in Saw III (2006), but since he was busy filming a TV series at the time, the idea was used for "Saw IV" instead,
The trivia items below may give away important plot points.
Betsy Russell was in the same outfit during the hospital scene after Jill loses Gideon, as she is in the interrogation scene. The camera slowly pans to the mirror to transition into the interrogation scene with Agent Peter Strahm. The sets were right next door to each other and Russell literally ran to the interrogation set for the transition to work properly.
This film marks the second time in the "Saw" series that someone other than Jigsaw (in this case Hoffman) says "Game Over" in the film's climax. The first time was in "Saw II" (2005) when Amanda delivers the line.
Tracy's character vanishes from the film without a specific resolution. Although it is implied she was kidnapped as well, this storyline is left ambiguous. The revisions in the script resulted in the character's shortened screen time. In the early drafts Rigg's wife would indeed be the target of the game and she would be suspended over the ice block trap. After Donnie Wahlberg agreed to reprise his role, the ice block sequence was rewritten and Tracy's character was dropped as the incentive of Rigg to search throughout the city.