User blog comment:Mobo85/"Phineas and Ferb: Mission Marvel" Debuts August 16/@comment-1896159-20130628214646/@comment-5358901-20130701041756

Per the tweet Moch posted, since that guy seems to have only been in Cal for a short time, I'd say either he was mistaken in his description (someone referring to the last ep of the most recent season might call it the "last ep") or it is a fake. But he did have a picture with one of the P&F guys IIRC (I forget which one), so it's probably not fake.

But the comments about a 2nd Marvel special make me really think that there will be a 5th season.