I doubt it.
They had so much trouble just trying to get number three in the works: first Disney greenlit it, then shelved it, then greenlit it again, then shelved it again, etc, etc, etc... not to mention, like everyone's been saying, number three was supposed to be the final chapter of the Toy Story saga, so a fourth one would pretty much serve no purpose. What could they do? Develop Bonnie and her toys' characters? That's just about all they could do.
Besides, Pixar is putting too much focus into sequels right now as it is: we just had Toy Story 3, now they're giving us Cars 2, and we have Monster's, Inc. 2 to look for... perhaps they're finally running out of ideas? I really don't know how they would do MI2, the first one was one of those movies that just didn't need a sequel (like Ice Age), all they could do with a sequel is perhaps Randall finds a way back into the monster world, and seek revenge on Mike and Sully.
I think what we need right now is perhaps another epic tale from Brad Bird, his Pixar creations have been really great, sort of like old Don Bluth movies: a bit edgier, a bit "mature", but still completely family-friendly with entertaining, yet heart-warming and inspirational stories.