A Goodreads user asked:
Where can I get this book? I got the first book, the third book, and the forth book for my birthday, but I didn't get the second one.
Aimée Marie Le Roi I found my copies online through a website called thriftbooks.com (totally not sponsored to say this at all), as the name suggests its basically an online thrift store exclusively for books. Its a really good place to find harder to come by or out of print books they get a lot of books from various used bookstores or thrift stores from across the country. Also, there is no fourth book in the series, goodreads lists one but that is actually a collection of two of the titles in one book
Silverin Er...about this fourth book, there are only 3 books in this series. I don't know which book you are calling the fourth but on the series page the fourth book called "Wizard's Castle" is actually an omnibus collection that contains both "Howl's Moving Castle" and "Castle in the Air."