Square Enix has registered URLs for ‘JustCause4', despite the fact that the publisher has yet to officially announce Just Cause 3.
Fusible uncovered the company's registering of URLs in several countries through URL proxy registrant Corporation Service Company.
Both Square Enix and Just Cause developer Avalanche Studios are mum on the registration, and in fact one of the last official mentions that the franchise would continue past the second effort came from a 2010 interview on our sister site Develop—where Avalanche head Christofer Sundberg shared that a third entry in the series was very likely.
Beyond all that, there's not much more to share. Just two weeks out from E3 though, so perhaps we'll learn something then.