At one point, the producers were considering using a version of the autopsy scene as the final scene in "Saw III." It was then retooled and used as the opening for this film.

The footage in the beginning of the movie that shows Jigsaw swallowing the wax-covered cassette was actually shot during production of "Saw III", but was not used in the final cut.

When Officer Rigg is walking through the school, before he finds Morgan and Rex in the rod trap, he walks by a dark room blocked off with police-barrier-tape. Originally, there was a scene that revealed this was the room where Troy from "Saw III" died, but it was cut, presumably for pacing reasons.

During the flashback scenes at the drug clinic, pay close attention. There are several characters from the previous films in a number of the shots. They include Paul (from the razor-room in the original "Saw"), Gus (who was killed by the gun-trap in "Saw II"), and the unnamed man who Amanda killed in the original film, among others.

The room the black guy goes into in the Alexander Hotel is room 261, where the rapist dies. California Penal Code Section 261 is the section of the law that defines rape and the punishment for rape.

Donnie Wahlberg (who plays a major role in this film, as he has in the past two films) was not secure until 2 days before shooting began. It was suggested that his character be replaced by another character just in case, but director Darren Lynn Bousman persisted, and eventually secured Wahlberg.

In each of the previous three films, the bathroom set had to be reconstructed from scratch when it was written into the script. (It was deconstructed each time under the assumption it would not be used again.) After "Saw III", the filmmakers decided not to totally destroy the set, as it might be used in future installments. And ironically, this was the first film in the series not to use the set, despite it being readily at hand.