Not even "They shot it a few years ago and just used it now," but more like when they were first planning season 4, they sketched it out that it would end after the college reunion, then got 10 episodes of material otherwise, and just decided to retool it as the Season 5 opener.

That way, the season would have ended with Issa and Molly slowly working things out, Issa definitively breaking things off with Lawrence, etc. Then by the next season, it feels less weird that Molly has a new haircut and they're having sleep overs, and Issa doing big things.