Saw 4 was the breath of fresh air that the series needed for so long. Saw 3 is the weakest of them all, because there's way too much sadistic scenes and way too little story.
The fourth film changes that. It doesn't has as many pointless gruesome scenes as its predecessor and it has an evolving and more believeble plot.
Picking up from where the third film left off, the film has two main storylines: one shows us that Jigsaw leaves a final puzzle behind after his death on the previous film; and the other is a flashback of what seems to be some time before the original film, showing a series of events that led "John" to lose all hope and turn into Jigsaw.
The film closes all the lose points left behind by all four. It has a really surprising ending which matches with the first.
As good as the original, if this would be the last of the series (I think not) it would end very well.
7/10