I haven\'t studied it a whole lot yet, but AFAIK the differences are:
The 2-disc version doesn\'t have a lot of special features (at least compared to the 4-disc Nov. version). It contains the version of the movie that played in theaters.
The 4-disc version has better sound (not 100% sure on that one, but think it was DTS-ES? Please correct me if I remember wrong) and better picture quality (not sure about this one either). It doesn\'t mean the 2-disc version is bad looking and -sounding, it just means the 4-disc version will be even better. Plus it has more special features. It\'s also 30 mins longer (which means that if you want the \'original\' version, you have to get the 2-disc version).
Isn\'t there also a 5-disc coming out? The 5th disc is a documentary I believe.
Again, this info might not be correct; it\'s just what I seem to remember.