My agent has had the book since January. She wants to sell the audio rights to Audible and hopes they'll buy it as an exclusive. If they do, the audiobook will come out first, probably 6 months before the paperback and ebook.
I am self-publishing book 4 because Tor only bought 3 books. My editor told me a few years ago she felt zombies are over. Traditional publishing is different from self-publishing. They have to move a whole lot of books to make it worth their investment, so if sales show zombies aren't selling well, they'll ignore that genre and concentrate on what sells. It was clear she wanted me to write a different sort of book for Tor, so I am currently working on a pitch for her with ghosts in it.
I can self-publish zombie stuff because my overhead is much cheaper and it doesn't take that long for the investment to pay off.
I'm as anxious as you are for a release date for After Siege. I will update this post as I find out more information.