Square-Enix has purchased a whole bunch of variations of the domain www.justcause4.com late last week, according to Fusible. The news is kind of shocking, when you consider that the publisher hasn't even announced that they're making Just Cause 3 yet!
Neither Square-Enix or Just Cause 2 developer Avalanche have spoken very much about the third entry in the series, which we now have to assume is on the way. News of a third Just Cause wouldn't be especially surprising: Avalanche head honcho Christofer Sundberg said that it was "very likely" that the studio would take another swing at the series back in 2010.
At that time, there were also rumors that the studio is working on a game based on the Mad Max film series.
Though little information about either has come to light since, it's entirely possible that the company is working on both games: Avalanche opened a second headquarters in New York in 2011 to work on a unannounced game only known as "Project Mamba."
Given how long it's been, it wouldn't be a surprise if we started to hear more from Avalanche about Just Cause 3 at E3 in a few short weeks. As for Just Cause 4... Let's take it one game at a time, guys.