While Men in Black 2 raised mixed opinions amongst MiB movie fans the third movie managed to do better and reportedly generated higher gross revenue. At this point the idea seems pretty much used to its full extent and the notion of another sequel would initially seem highly unlikely to happen. Yet, Hollywood is reportedly preparing to reboot the movie.
Such moves are not unheard of. Take the example of Sony which started over from scratch with the movie The Amazing Spider-Man. For Men in Black 4 most of the details are still unclear but one thing is known. The movie already has screenwriter Oren Uziel onboard. It is not known if Will Smith will be involved in Men in Black 4 or if the movie will go forward without him.