I doubt they'll go to a 4-3 base, but I do expect them to mix in a little more 4-3 than usual, given that DL depth is a strength and LB depth a weakness.
I think Sullivan is a guy who's always had the talent but has serious character/motivation issues. Hopefully, a change of scenery will help, and the realization that he got kicked off his old team in what was essentially a dump move will serve as a wake-up call. He's made enough plays in the preseason to convince me that he can be, at worst, a serviceable body along the DL.
The WR situation is troublesome, but as you say, the rest of the offensive personnel is as dynamic as it's been during the Pats' 3 for 5 run. Watson came on strong at the end of last season, and he looks ready to become a force in the passing game.