I'd like another one too, but Westerfeld made it pretty clear that he was done with the series. I think he wanted to right more, but it just wasn't as popular as it should have been. Well, one thing for that (starts sharpening knifes and starving Throwing Sharks) I'm going to go murder some Twilight fans.
Nah, kidding.
Though, honestly, where would he go with the fourth book? I doubt you guys want to read about Alek and Deryn doing stuff for Barlow. At least, I know I don't. All the good stuff (read: THE ENTIRNY OF WWI) has been cut off thanks to the Leviathan. There might not even be a second World War in this one, which sucks, because then my Grandpa will never be deployed to the Phillipines, never have the life-changing experence he had there that made him marry my grandma, which will mean my mom will never be born, which means I WONT BE BORN. Thanks Westerfeld.