if the stories dont' suck then i dont' see why not to keep making them. if there is just as much story, effort and creativity put into each one then yes, i will watch them and probably buy them. they have the same director and story writters for the majority of the series, and part 5 is going to have a new director, but this guy has been attached to these movies since part 2. i certainly dont' want to see them draw out jigsaw's death through a part 5, they need some new stuff, but maybe they will actually make some good choices that will lead to some originality in the future incarnations.
i'm hoping there isnt' a pattern forming for these movies though.
part 1: 2 guys in the same trap for the whole movie
part 2: people going through a house finding situations where they have no choice but to die or involve themselves in the traps
(so far so good)
part 3: one guy watching or helping people that are stuck in traps, of which he has some personal tie to
part 4: one guy running around town watching or helping people in traps, one of which that we know he has a personal tie to
i just hope that part 5 doesn't have a similar situation to the previous films