Is 3 enough?
Personally, I think that "The Matrix" should end with a trilogy. If more movies are made about the matrix, I think that it would fizzle out and many people would get tired of the idea. However, I'm sure that there will be other films that use this theme of humans not being in control of their lives, and have a chosen few trying to change this.
Of course if someone does continue on "The Matrix" at least give it a few generations.