What's the difference between 3 and 4?
The easiest way to think about it is, who would Tampa Bay take that will now be available? And not necessarily available to us, but available to anyone who might want to trade up to get.
Alot of mock drafts have Tampa taking Calvin Johnson, and that certainly makes sense. With the great things they say about that guy, I would complain too loudly if we took him, even if he could be another Braylon Edwards. Another likely target for Tampa is whichever QB is still on the board, probably Brady Quinn.
Both Quinn and and Johnson are essentially irreplaceable prospects. While guys like guys like Joe Thomas (OT), Alan Branch (DT), and Adrian Peterson (RB) are pretty solidly the top guys at their positions, they all have substitutes who might look like viable options. So if Houston wants an OT and they're convinced that Tampa will take Joe Thomas, they could be content that at least they will have a shot at Levi Brown. In the same way, there are other DTs and RBs so that other teams wouldn't necessarily feel terrible about missing out on the top guy.
But if you want a QB, and Jamarcus Russel is off the board, there aren't any good alternatives to Brady Quinn. So if other teams are convinced that Quinn will go to either Tampa or Cleveland (and from the rumors floating around, the Browns are doing a good job showing that they're interested in a QB), the #3 spot could look like a good spot to trade up to. And while there are some other good WR out there (Ted Ginn, Dwayne Jarrett), neither are really in the same league as Calvin Johnson.
So, if we're going to get a blockbuster trade for our pick, being ahead of Tampa is a good place to be.