I don’t see a 4th listed on Goodreads, but I wish there was!
Sophie, Howl, Calcifer, and even Morgan, are up to tricks and trouble once again, but again, they aren’t really the main characters! (They’re presence is still as powerful though! Never fear!)
This time it’s all about a young lady named Chairmain, and what happens while she’s looking after a Wizard’s house for him. Beyond that, it’s a lot of Howl-esque sorcery and Secrecy!