When I went to see this at the cinema, I had assumed the ending of Part 3 was setting up the plot up for this one, and we'd see what became of the sole survivor from that. Instead, we see that despite the death of Jigsaw, another set of games are going on, thanks to a mysterious accomplice, and it isn't Amanda (the girl from the previous films).
This time it seems SWAT Captain Rigg (Lyriq Bent), who you may remember had minor roles in the 2 previous films, has been singled out for gameplay, when the new Detective in charge of the murders, Hoffman (Costas Mandylor), who you also may remember was introduced in the last film, is abducted.
Rigg, who has been obsessed with finding detective Matthews (Donnie Wahlberg) ever since his disappearance in Part 2, finds himself being forced to go through a series of tests, involving various seedy characters in order to save Hoffman and find out what happened to Matthews.
But with a trail of bloody corpses following the wake of Rigg's 'tests', the FBI jump on the case, with 2 of its agents Strahm (Scott Paterson) and Perez (Athena Karkanis) trying to find out what he's up to and what's happened to Hoffman.
Featuring plenty of gore, including eye gougings, knife slashings and various impalements etc, and numerous plot twists, this is an excellent follow up to "Saw 3". We also get see a lot more of how John Kramer (Tobin Bell) became the Jigsaw killer.
Overall a very good and entertaining horror film, but the series is now at the point where you really need to see the previous movies to understand what's happening.