In the early stages of development there were only going to be three characters
When did the fourth character come into play?
It was soon after the game had made it further into development that occured.
Why did they add a fourth at the last min?
There was a love interest created for Cloud.
Cloud is only in Final Fantasy 7
They had crafted Tifa earlier on and then permanently added her later.
I see now that you are referring to FF7, I was referring to the series as a whole.
Yeah, I probably should have mentioned that was the development I was talking about. 
So they went from 3 characters to 7?