Saw IV follows an investigation and a series of traps that run side by side, and both end at the Gideon Meat Factory. The film begins with an autopsy on the dead Jigsaw killer. A microcassette is found in his stomach, and its message for Hoffman says the games have only just begun.
Officer Rigg, a tireless cop who cannot stop trying to save others, is forced into a series of traps. The stakes are the lives of the people he is trying to rescue, including Detective Eric Matthews, who turns out to be alive. Eric appeared in saw 3, where he got out of the bathroom trap he had been left in.
Agents Strahm and Perez study the case and try to stay one step behind the new game. Jigsaw's widow Jill is questioned and talks about a past miscarriage that shaped John Kramer. Flashbacks fill in how Kramer became the man behind the puzzles, and together with Jill's account they give the film its emotional spine.
The clues lead the investigators to the Gideon Meat Factory, where the final test takes place. Rigg must choose whether to follow the rules of the game or break them in order to save a victim. It is revealed that Hoffman is another apprentice of Jigsaw and has been involved in the games. He seals Rigg and Strahm inside the factory, leaving them trapped.
In my opinion, the structure is what sets this entry apart. It splits its attention between the police procedure, the personal history of John Kramer and the game Rigg is forced through, and the reveal at the factory pulls those strands together.
