Despite what might have seemed like a less than typical/expected domestic response to MIB3 last summer (it made about $260 million less than the first; and about $100 million less than the second - adjusted for inflation), the film still managed a worldwide gross of over $624 million! Keeping inline with the changing times, the bulk of its box office (more than 70%) came from overseas.
It helps when you have an international superstar like Will Smith, and a known brand.
So maybe then it shouldn't be a surprise that a 4th Men In Black movie has be greenlit by Sony Pictures, with Oren Uziel (a relative newbie, according to his IMDB page) tapped to pen the script.
Said to be in its early stages of development, it isn't yet known whether Will Smith will return for a 4th movie. I recall his ambivalence in doing a 3rd, saying that he'd do another one if the fans demanded it.
And I guess with a $624 million worldwide gross (on a $225 million budget), one could very well argue that the fans wouldn't mind a 4th movie.
Count me out though. The first was enough for me. I dug it, was entertained by it, and was happy with the way director Barry Sonnenfeld wrapped it all up. The second and third films in the franchise did little for me.