SI has no offical relase date but my the consensus has been for a while it will be late November or early December. That is not offical though and the release date may end up being in Jan or Feb or whenever, it really depends on when the game is finished.
However on Oct 24th, they are releasing a magazine preview of CM4, which all you folks in Europe (espically England) are lucky and will be able to purchase. This will be the first offical word about the game, the first discussion of the features and tje first screen shots
to answer your question dpf, this is an entirely new game so it won't be an extension of CM3. Obviously the game will be similar to the previous versions in many ways, but this one will be different (including a 2D match visulation engine).