As Creep said, the films slowly reveal more about Jigsaw and his backstory with each successive release. Saw IV in particular is the most revealing so unless there is no more to come, Saw V should offer the final pieces of the puzzle (pun not intended).
It won't as they have planned to release a new SAW movie every year until about SAW 10 suppposedly, they have scripts for them all, already in pre-production. On a side note, I am down to be interviewing Tobin Bell for a forthcoming Podcast...
Cool. I know what you mean about the Saw films though. I'd never watch it in the cinema but if I see them going on a 4 for £20 deal somewhere I pick them up.