The iPad 3 is such an incredible device (just like it's predecessors). The iPad 4 will not be coming out till next year Sept.
Based on the the new Mountain Lion OSX, apple are going for evolution and integration rather than revolution.
The iPad 2 and 3 are very similar except for a few differences noted above. The only thing "new" in the iPad 4 will be the further integration of ios6 (which is coming soon)! So don't expect holographic screens yet !