So, it's hard to tell if it's needed or not before time. But all that I can say, it's that I definetely want a Transformers 4!
I wouldn't discard the possibility that this was project already in development/on-hold, just waiting and relying on how well would TF3 would do. And, following this point of view, you can imagine that saying that this was the last movie of the franchise can make more people watch it. Not that I really care about that