So I finally had a chance to go see the movie, and like the other movies I wasn't disappointed, I would definitely recommend seeing the first three before this one, or else you'll be completely lost throughout the movie.
I don't want to give any spoiler though its very tempting.
But to answer some questions, yes there is a story line, the bloodiness and gore I would say is about the same as the previous movies.
I thought it was was another very good movie, they found a way of connecting all four movies together, yes some parts were a little confusing, at least for me they were.
Definitely one to see if you've like the previous ones.

 

This probably violates a rule or something. But I have never seen any of the Saw movies.

As I said, I only saw the first one and I disliked it. I thought it was a waste of film myself. *shrugs*
Everybody is different.