It looks like the final shattered dimension will involve a black-suited Spider-Man.
GameFly revealed, and then quickly removed, Spider-Man: Shattered Dimensions' box art, which showed a black-suited Spider-Man as the final character.
The dimension could be a symbiote dimension or some sort of throwback to Marvel's Secret Wars mini-series from 1984, which first introduced Spider-Man's black suit. In the teaser at the end of the video from the previous dimension reveal, the popular rumor was that it was a symbiote dimension, so this revelation is in line with previous speculation.
Specifics about the fourth and final dimension will be detailed soon as IGN is currently promising a reveal on July 21, and there is a Comic-Con panel scheduled on July 23. The other three dimensions are Amazing, Noir, and 2099.
Spider-Man: Shattered Dimensions comes out in September 2010.