The new edition is a total re-write and significantly expanded from the 3rd edition, it is certainly written so that it is an essential buy for anyone who has the 3rd edition. I got a few sneak peaks, I wasn't going to pass on that, which left me very keen to see the whole thing in the flesh. The book is due out this year and is already listed on Amazon (UK) with an early December release. Although I think Martin hopes it will be out a month earlier than that.
Here is a picture of Mr Edge, which I snapped over his shoulder as he was showing me some of the images from the section on schooling fish.

Alex