I think Men In Black could go on forever. Of course, that should only happen if the stories make it necessary. A main factor of the appeal of the first movie was that it was completely original. After that, the stories can be original, but you can't re-capture the newness and overall original-ness of it, being introduced into that world. Since they wrapped things up nicely in the third movie and since the fourth one involved other characters and was pretty average, they should just leave the original universe alone.
I think it could be a good idea to reboot it in the future, slightly change the tone to make it its own, and have the release of the films be more consistent. A big picture of multiple films should be in mind up front so that they don't vary in quality. I'm interested in seeing what might happen.