As someone sitting there front-row in the cinema on the release of Saw I, II and III; i will no doubt be there for IV and V when (if) they are released.
Remember there were a lot of open ends in Saw III. To avoid spoilers I'll be sparse on the detail (these apply over the series so far):
Amandas letter, Eric's kid, 'the girl', Lawrence Gorden's (and his wife and kid's) fate, the parent of 'the girl' and so on.
Regardless, they've delivered us 3 good installments of this series so far, and I expect they can bring out another good one. Anyway, we only have a year to wait and see.