You can't "jump the shark" in the first season. That implies a downward slide that you can't recover from. If they are now into the 5th season, then they recovered from whatever you're talking about. (Even Heroes jumped the shark more or less in the 2nd season.)
Well, some people thought they jumped the shark last season, and while it was a somewhat lackluster season, they didn't go completely off the deep end. And they actually finished their arc with plenty of closure. This season had potential early on, but it was really turning into scattershot, with that same "we didn't plan this shit and we're writing episode-by-episode" bullshit that I loathe with most series. It was especially obvious the further into the season you went, with more loose ends and plotlines than an episode of Game of Thrones, except they don't really go anywhere special.
But, the real turning point was the finale. You can't fuck up the main character like that and NOT jump the shark. Point of no return. It's going into Heroes territory now, which is fucking scary. Nah, even worse, it's going into SEAQUEST territory. I mean that level of "painting yourself into a corner" fucked up.
HBO will probably be grateful enough to give Alan Ball a final season, but mark my words: the next season will be the last.
Rosalina: But you didn't.
Robert: But I DON'T.
Rosalina: You sure that's right?
Robert: I was going to HAVE told you they'd come?
Rosalina: No.
Robert: The subjunctive?
Rosalina: That's not the subjunctive.
Robert: I don't think the syntax has been invented yet.
Rosalina: It would have had to have had been.
Robert: Had to have...had...been? That can't be right.