From there, Michael will be given immunity by the government to rescue Sara and destroy The Company with the help of brother Linc, Sucre, Bellick and Mahone. A big part of the season will involve the gang having to break in to a maximum security building where the villains reside, including uberbitch Gretchen. Meanwhile, T-Bag will become relevant because he still has possession of a "vital clue" -- which must be Whistler's bird book...
It all sounds quite interesting, although I'm not sure how it will all fit together -- what with Sucre, Bellick and T-Bag stuck in Sona prison at the end of season 3. And didn't Mahone join forces with baddie Whistler, too? I'm predicting the first half of season 4 will revolve around getting Sucre (and by extension Bellick and T-Bag) out of Sona, before this year's "twist" of breaking in to The Company's HQ becomes the second half focus. But that's just a guess. Where are The Company based if the US government can't just storm in there, though? Hm.
As I mentioned before, I'm still not jazzed about Prison Break's return (not helped by the silly about-turn over Sara's fate), but this show has the uncanny ability to get your pulse racing. It's never been particularly intelligent post-season 1 (as it used up its unique ideas there), but I like the cast and there's something refreshing about a show that isn't pretentious and just wants to entertain. Pure and simple. SPOILERS END!