I think that is very likely and a pretty reasonable guess. The only thing that makes me think it might be the Ada scenario is that it was mentioned in that early leaked information and it seems like everything else they predicted has now been confirmed.
Maybe the 4th unlockable scenario is what you suggested and there will also be an Ada scenario upon game completion or as DLC? Or maybe Ada will just be an unlockable character to play in that 4th scenario...maybe it turns out that her clone is actually responsible for everything Chris is blaming on her and she joins up with Chris and Leon? You can choose which of the protagonists you want to use for the final scenarios - Chris, Leon, Jake, Ada, or maybe even their partners?
Now I am just wildly speculating and I am going to stop.