If you look at all trilogies, (The Matrix, Star Wars, Indiana Jones, Lord of the Rings etc etc etc...) these are always a success. As soon as directors/filmakers start with a fourth film the storyline and passion of the original films decline known as "milking it" (Aliens, Superman, Lethal Weapon and so on...)
I think they'll be making a mistake if they do make a 5th. The first three were great the fourth wasn't so. They may aswell make the fifth but it won't be as good as the original 3.