The article only says FOX is considering the option for a fourth Seth series; they haven't decided on it yet. This might not even go anywhere.
And if it does, who's to say that Seth will necessarily have four shows going at once? Maybe they'll wait until one of his current shows ends. "American Dad" seems to be on the thinnest ice right now, what with its 9:30 death slot, low ratings, a total lack of merchandise, and an upcoming midseason hiatus, so if any of Seth's shows would get the plug pulled, it'd probably be that one - which stinks, because it's the funniest thing he's got right now.