Prison Break : Whistler's bird book central to season four

So you thought Michael Schofield's tattoos were tricky in Prison Break season one.

Well, you ain't seen nothin' yet.

That's the verdict of series producer Zack Estrin, who's dreamed up a whole new catalogue of twists and turns for the upcoming season four.

And for catalogue, read Whistler's bird book.

"Just like Michael's tattoos in the first year, the book has this season's mysteries in it - riddles, clues and puzzles," Estrin says.

"Michael and the team will need to get the book in order to complete their tasks. Of course, that won't be easy!

That's an understatement. When last seen, the battered bird book was in the grubby hand of a certain psychopath who's now kingpin inside Sona prison.

Yep, that would be T-Bag.

And yep, they're going to have to break him out.

"T-Bag is the keeper of that book," admits Estrin. "How they find him, and what happens when they do, becomes an integral part of the plot."

We'll also learn more about what Whistler is really up to.

"Obviously Whistler was a bit of a mystery in season three," Estrin adds. "You will find out exactly what he was up to. His real agenda will be revealed."