It says a lot about how self-aware Hayate the Combat Butler is, that when the series starts a major arc (where a TV series would start a new season), it lampshades
this by calling the chapter "If This Were an Anime, the Opening Would Change!"
That title could basically be the title for the volume itself. For the past 3 volumes of the manga, we've been hanging out with Hayate, Nagi, Isumi and Wataru and bouncing back and forth between their various domiciles. While we've gotten a certain degree of action in the volumes, particularly where the schemes of Nursing Robot Eight were concerned, the series has remained somewhat staid in its status quo. That changes here. Break's over, and now Nagi returns to school, which means now we get introduced to her school, the Hakuo Academy, and some of its students and faculty and in particular, their butlers.
Frankly, for a series called "Hayate the Combat Butler", we haven't necessarily gotten a lot of battle buttleing in earlier volumes. Yes, Hayate's fought Eight, and he's fought the Yakuza, and he's occasionally fought Tama. However, we haven't really seen that many other battle butlers. This volume changes that up - particularly through setting up rivalries between Hayate and some of the other butlers, all who generally have nice personalities.
If I have any complaint about this volume, it's that the butlers get more screen time then their masters - which in turn leads to something of a flaw with the show - aside from Nagi and Wataru's households, the focus of the supporting cast is either on the masters or the butlers, never really both. We never really get the names of Isumi's butlers. As far as we know, Aizawa doesn't even have a butler. Here, we get two butlers and their masters introduced. Himuro and Taiga have an interesting master-butler relationship that's different from the traditional master-butler relationship, to the point that it's practically reversed - but aside from seeing Taiga tossing cherry blossoms for Himuro, we never go any further than that. Azumamiya and Nonohara are also introduced this volume, but we don't see Nonohara at all, and Azumamiya basically only appears for a page.
Hopefully, these new characters will get fleshed out further in later volumes, as I'd prefer to avoid the Butler of the Chapter. Still, this volume causes a nice tonal shift, which future volumes can only build off of.