Brotherhood isn't meant to complete any part of the main story line IIRC, and the developers said for a while "This will NOT be ACIII but a separate game"Wirelessly posted sponge:: Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_3_2 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8H7 Safari/6533.18.5)
The real trilogy. You play AC and then AC2 picks up right at the end, and then Brotherhood jumps in exactly where that ended. You could play them as one continuous game. Do the others add anything integral to the plot? No, since I've never played them and am not missing anything.
This series may suffer from too many expansions and adaptations, but the core 3 games so far are solid as hell and fairly consistent. If this is the final chapter that picks up where Brotherhood left off, it should be great. They don't need to do much tweaking of gameplay, features or graphics, just new plot developments.
And I doesn't need to be numbered, as I don't believe it will be introducing a new character. This should be focusing on Desmond and be the chapter he was preparing for with the other two stories within the story (Altairs and Ezios).