Ah, so that's Michonne. And what a pleasant little flower she is.
Vol. 4 of The Walking Dead is my favorite of the series so far, although I hesitate to actually use that word 'favorite' on something this disgusting. Maybe it's more appropriate to say that it's the most well-told volume so far. Our band of survivors have been slip-sliding down the slippery slope of life during the zombie apocalypse. Now that they've found shelter, insulated themselves from outside harm, and most of their short-term worries have been alleviated, they can start to worry about the bigger picture, like for example, coming to terms with the fact that living through the zombie apocalypse necessitates a certain amount of adaptability. Vol.4 mostly consists of characters reaching their emotional limits. Relationships are strained and tested (and broken) and certain choices are made that can never be taken back.
The climax of the story comes with a brawl between Rick and Tyreese that ends with Rick being deposed as 'leader,' and subsequently going apeshit all over the remaining survivors. The last line of the book is a powerful one -- and you can tell this is a moment that Kirkman has been working up to since he first named the series. "We ARE the walking dead," screams Rick. "WE are the walking dead."
It's a little heavy-handed, but an effective moment nonetheless. From here on out, the survivors will presumably stop trying to get things back to the way they were -- there are no rules or laws anymore, and the most important thing is to survive.