This is going to sound like a spoiler but, hey, it's how the film starts. Jigsaw (Tobin Bell) is dead (told you) and Lt. Mark Hoffman (Costas Mandylor) is called in to analyses the corpse after an autopsy that reveals he hid tapes within his own body, making sure the game continues after his death. SWAT Commander Daniel Rigg (Lyriq Bnet) is abruptly kidnapped and thrust into another one of Jigsaw's trials while FBI agents Peter Strahm (Scott Patterson) and Lindsey Perez (Athena Karkanis) are called in after another cop is killed in one of Jigsaw's game. The fate of Eric Matthews is still unknown as Hoffman finds out just what has happened.
Saw IV suffers from a big flaw. As always, there are two narratives running however the one that follows Rigg is a problem mainly because Rigg just isn't a very interesting character. Previous test subjects like Dr. Gordon and Jeff have been the best characters in the film however Rigg not only doesn't have much to work with but is also overshadowed by the other plot that follows Strahm and Hoffman, two much more interesting characters. So half of the film works quite well while the other one falls short of being interesting. All the best moments in the film are all from Strahm's plot. The climax of Rigg's is good however but that's only because of the twist, as per usual, which is only set off by Rigg's stupidity. Yeah, not a great character if his lack of rationality costs people their lives.
I think what really works about this one is the twist. I won't go into too much detail but it's pulled off very well and makes great use out of de ja vu similar to how Pulp Fiction does. It brings about an idea that I wish more films did well. Saw IV and Pulp Fiction really are the only films I can think of that pulled if off well. The use of visuals and sound in both films make the audience go "ooooh! I get it now!" and those eureka moments are what make the Saw films worthwhile. As for the traps, they're okay I guess. They do their job but pail in comparison to earlier and upcoming traps. Not the worst but nowhere near the best.
Saw IV is okay at best. It suffers from the mind set that it's the film AFTER the trilogy like Pirates of the Caribbean: On Stranger Tides. While it's not as pointless as On Stranger Tides, I am left wondering what Saw would be like if it was just a trilogy. Yes, we would miss out on Saw VI but there would be less dull and dragging moments in the franchise. The new characters are good but the protagonist can't hold the film up. It's not the worst yet not the best. That's the best way to sum it up. It's in the middle somewhere. Arguably the most forgettable in the series.
The most forgettable entry to the franchise that, while fairly dull, it does have a few things going for it...just not enough to hold it up.