It makes perfect sense that the books have had that rhythm, too. Book One had to do all the heavy lifting—establishing the rules of magic, the tone, and introducing the cast. Book Two got to reap the rewards of that setup with the body-swap chaos and the cult. Book Three had to do the hard, emotional work of addressing the trauma, setting up the Ministry bureaucracy, and dealing with Edgar's looming threat.
But Book Four? Book Four gets to take all that established lore, all that hard-earned emotional growth, and just play with it.