The movie features scenes of violence and torture throughout. It was originally rated NC-17 by the Motion Picture Association of America. The rating was later changed to R. An uncut and unrated version has also been released in the United States. Saw IV was banned in Malaysia and was released with a special warning in France.
Saw IV performed reasonably well at the box office. Having been made on an estimated budget of $10,000,000, it went on to earn more than $63,000,000 at movie theaters in the United States.
A wax-coated tape is found in John Kramer's stomach during his autopsy. It is given to Mark Hoffman, who learns that he will not walk away untested.
The scene cuts to Trevor and Art Blank, who awaken chained at the neck to a winch in a mausoleum. Trevor's eyes are sewn shut, as is Art's mouth, preventing communication between them. Trevor's panic activates the winch, and Art kills him after a brief struggle and takes the key to free himself. Allison Kerry's body is found by police four days after her death. Hoffman is met at the scene by FBI agents Peter Strahm and Lindsey Perez, who Kerry had previously contacted, after sending Lt. Daniel Rigg home.
Strahm soon becomes suspicious of Rigg, who is convinced that Eric Matthews is still alive. Rigg is abducted in his home that night, and awakens to discover that Matthews is alive, that he and Hoffman have 90 minutes to save themselves, and that Rigg must undergo tests to discover what it truly means to save a life.
Rigg saves Brenda, a madam, in his first test, but is forced to kill her when she attacks him, as she believes he will imprison her if she doesn't kill him. In his next test, Rigg enters a motel and forces Ivan Landsness (Marty Adams), a serial rapist, into a trap where he must choose between blinding himself and dismemberment. Ivan fails to blind both eyes and is killed by the trap. In his third test, Rigg enters a school and finds Rex and Morgan, an abusive husband and his wife, impaled together by spikes in a harness.
Strahm and Perez alternate between investigating each test scene and interrogating Jill Tuck, John's ex-wife. While married to John, she had been pregnant with his baby, which he was to be named Gideon. Jigsaw's puppet and its tricycle were meant for Gideon. She miscarried when Cecil Adams, a drug addict, was robbing her clinic. The agents also learn that the motel room was rented to Art Blank, who vanished two weeks previously, and that they are the next two targets.
Throughout Rigg's tests, Matthews and Hoffman are overseen by a man revealed to be Art. Matthews is held atop an ice block by a chain-noose, and Hoffman is strapped to a chair with an electrode at his feet. They are balanced by a seesaw: if Matthews falls after too much ice melts, he will be hanged and Hoffman will be electrocuted. At the third test scene, the agents learn that all three victims were represented in court by Art, who was also Jill's lawyer. After Perez is hospitalized by an exploding puppet, Strahm furiously questions Jill, now certain that Art is the accomplice. Jill recounts that John's depression following the miscarriage ultimately ended their marriage, as well as his property development work with Art. After his suicide attempt, John began his work and targeted Cecil first. Cecil was to push his face through several knives to hit a switch and release himself. The chair collapsed via freeing himself but Cecil died after falling into a tangle of razor-wire trying to kill John with a large piece of the chair. Strahm connects her story with the Gideon Meatpacking Plant, the location of Rigg's final test.