From my point of view, it's not that Jennings is overrated. Its the fact that past the top 4 WRs (AJ, Calvin, Roddy, Nicks) what you have is a glut of receivers that all have pretty good question marks. Any of them could produce in the top 5, but none of them are safe enough to go ahead and put them up there.
So: (in whichever order you prefer)
AJ-
Calvin-
Roddy-
Nicks-
-------------
Jennings- possibly hurt by Finley. Really was more hurt by poor offensive line play (Rodgers getting sacked 50 times), which handicapped Rodgers' ability to let downfield routes develop, cutting into Jennings' production in 09. This has been remedied somewhat, as the line played better last year. Finley is just as likely to draw attention away from Jennings deep routes by pulling safeties toward him, since he's a total mismatch for virtually every LB in the league. They're still going to throw the ball more than almost any other team in the league. As acting #1 target for nearly an entire season, he has an established rapport with Rodgers that Finley does not have yet.
--------------------
DeSean Jackson- relies heavily on big plays for his fantasy points. Doesn't catch the number of passes that the big 4 do, or even Jennings for that matter. Is a marginal injury risk because of his size. Maclin continues to emerge as a 1a in Phil.
Mike Wallace- The new #1 in Pit, is a Jennings-like player with great speed and downfield ability. Can take any post to the house. Still didn't catch a large volume of passes, and big plays contributed to his totals.
Vincent Jackson- Will he hold out again? Is he the WR1 in SD, or is Floyd now? Will he be out of SD? Playing with a lesser QB would drop him to the bottom of my top 10.
Larry Fitzgerald- As of right now, there is still no viable QB in AZ. Fitzgerald could feasibly leap back into the top 5 with even adequate QB play, but there's a lot of uncertainty here.
Reggie Wayne-Physically in decline now, and with more weapons than Jennings around him to share targets with, I can't see him getting into the top 5 again, but will be drafted high because of his name.
Miles Austin-Is he or Dez the guy? Can they coexist and put up top 12 WR numbers simultaneously? Witten is still here.
Dwayne Bowe- With Weis' departure and a tougher schedule queued up for this season, it'd be hard to imagine him getting back into the top 5.
So as you can see, it's not a question of declaring Jennings deserving to be the 4th-5th WR off the board definitively. Its more an issue of, with the exception of the AJ, Calvin, Roddy, Nicks, Jennings is as good an option as you're going to have. The other WRs in this tier have just as many, if not more problems/questions surrounding them. This is why he's WR4-WR5.