I've had a debate with myself for a few years now over which entry is the worst in the Saw series: IV or VII/3D. After rewatching all the films in a three day period, I can safely say the fourth film is the absolute worst. Jigsaw is dead. Sort of. Kind of. We have more paradoxes caused by IV than the entirety of Doctor Who combined.
So Jigsaw is dead (sometimes, not always, depending on what part of the story you're in). A detective who has been on the case from day one is the new victim, forced to get into the mind of Jigsaw and launch the traps himself. Everyone is trying to find out who Jigsaw's assistant is, since Amanda is now physically incapable of doing things we actually saw her do in flashback in II and III, and all signs point towards the detective being tested.
Saw films consistently fall apart in every cop scene. It's a fact. The only remotely successful cop scene happens in the first film (the workshop traps) and it's really a "demonstrate Jigsaw's foresight" moment more than a cop moment. To call the police force investigating the Jigsaw murders dumb cops is an insult to Chief Clancy Wiggum.
Despite their main suspect literally having an alibi for every night except for the one he's tested on, everyone on the force thinks he's clearly the accomplice. One brave soul suggests there might be a framing, but they never speak of it again. It's hive mind around one suspect who makes no sense with the added red herring of Jill Tuck.
Jill Tuck, the greatest character in the series, is John/Jigsaw's former wife. She runs a free clinic specializing in methadone treatments and accidentally inspires the Jigsaw monster when she lost her unborn baby in an accident with a junkie. The police constantly pull her into the interrogation room for the rest of the films, demanding Jill (a never-better Betsy Russell) reveal everything about the Jigsaw killer even though she divorced John long before the plans for the murders even started.
While the police circle clear dead-ends for 90 minutes, the poor detective in Jigsaw's game faces an even worse fate. Not until VII/3D do we ever see a game concept as poor as this one (that would be the garage trap, which is mind-numbingly stupid). One by one, the detective is forced to set off traps and leave victims to fend for themselves or face punishment. And another pair of cops are trapped on a rig that will hang one (he's standing on a melting block of ice) or electrocute the other (sitting on the other end of a teeter totter near electrical cables that will fry him if the other cop dies). It's...dumber than it sounds.
That's the perfect way to describe Saw IV: it's dumber than it sounds. It's the first time neither Leigh Whannell nor James Wan had anything to do with the series and it shows. It bares no relation to the original film or even the first two sequels. The twist used to justify the change in tone just makes the whole thing worse.
The entire Saw series received a beautiful Blu-ray release. Otherwise, Saw IV can be rented from all the major digital platforms.