The fourth one? You mean another novel of Pullman's the barely mentions the name Lockhart? The Tin Princess, I assume?
Anyway, I read these a couple years back - wonderful books, like everything of Pullman's, but I don't think they're for children. I mean, there is an awful lot of
possible murders, and a WHOLE LOT of drugs - mainly, opium. Add crime lords and other random bad stuff in there, and the trilogy is not for kids.

p.s. Already a Pullman thread here:
http://anthonyhorowitz.com/msgboard/index.php/topic,2830.0.html