For fear of repeating myself too many times, here's the quick rundown on the overall strengths and weaknesses of the series before delving into Book 4-specific details: The strengths are the gripping plot and the developed aspects of the characters, whose interesting actions move the story along. The downsides are the reliance on cliches/stereotypes that take away from the surprise and the lack of complexity in some of the characters. These hold true both for this book as well as the entire series. 
 In Fire and Ash, Nix and Benny's relationship complicates in a very good way. (view spoiler)[Their decision to take a break - which makes sense both for the characters and within the context of real emotions - allows for romantic complexity that enhances the story. The ending, and Nix's muffin peace offering, is cute but a little cliche and unsurprising; the pat ending denies any further complexity. (hide spoiler)] 
 Overall, the series was an enjoyable read. Both interesting and compelling, the story pulls readers in. Definitely a great read for zombie fans!