I dunno. We'd have heard something about a fourth one by now, wouldn't we? Plus, given that they're all time-travel movies (of sorts, taking into account the first one's suspended-animation gimmick), they've been to the '60s, the '70s, and the '90s - I guess the question is, do we really want to see Austin Powers trying to adjust to the '80s.
B'sides, Myers has been adding a multiple role with each sequel, so I can't say whether he'd want to drag himself out to bring in a fifth one for another AP movie. And let's face it, between this franchise and Shrek, Myers is already practically typecast again...the only other memorable thing he's done is Wayne's World.
So I'm gonna say no at this point.