The seamless transitions between separate scenes in the movie 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.
Betsy Russell was in the same outfit during the hospital scene after which Jill loses Gideon, as she is in the interrogation scene. She the camera slowly pans to the mirror to transition into the interrogation scene with Strahm. The sets were right next door to each other and Betsy literally ran to the interrogation set for the transition to work properly.
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, 'Liriq Bent' was the victim. He actually managed to remain in a character a couple of seconds before bursting into laughter.
Saw IV (2007) was given the green-light before the third movie 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."
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.
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 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. 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.
The trivia items below may give away important plot points.
The trademark flashback montage at the end of every film in the series is supposed to tie some loose ends of the plot by providing explanations using earlier scenes as hints to what happened. In the revelation montage for this film, the flashbacks establish Hoffman's presence as an unknown accomplice, like for example the fact that he was able to lift Kerry's body. Interestingly there is a scene that is excluded from the montage and therefore is left unexplained. When Agent Perez asks Hoffman why the teddy bear is in his possession he replies that it is a short story. This hints that the toy is a gift to soothe Corbett, the kidnapped daughter of Jeff, for the short time period that she is trapped in the lair. This also provides some insight to the intentions of Hoffman since he planned to save the girl and establish himself as the hero something that is revealed in the next film.
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.