This news comes from Hitfix, who have no idea who is directing this sequel. Jay Roach, who directed the first three installments, is expected to return, but its to early to make that an official call just yet.
Austin Powers 4 will come more than a full decade since the last movie, Austin Powers in Goldmember, which hit theaters in 2002. The character has a whole new time period to react to now.
It isn't known when the film will go into production, but its likely that we won't see Austin Powers 4 until, at the very earliest, 2013.