I went into this one with low expectations, but I was floored by the end -- a huge improvement over Part III, with the series' most compelling pair of protagonists yet: Rigg, a seemingly stable, honorable detective-turned-accomplice disgusted with the city's moral failures, and Jigsaw himself, whose character deepens with a badass vigilante origin story.
And then Luke from "The Gilmore Girls" is added to the cast as a hard-as-nails federal agent! What? "Saw IV" features all the hallmarks of the previous (and most of the subsequent) films in the series, meaning that it's mostly very boring and gray with occasional flashes of red.
Some brilliant transitional effects done in camera with no CG. Very complex plot, took me a couple of views to figure out what was going on. Reveals a lot of things that were planted into the 3rd movie, the twist threw me for a loop a bit but i ultimately liked this one.