I do intend to update it for a fifth edition. I originally was waiting until the DocBook assembly stuff was completed before updating it. The TC process has taken longer than I expected to finalize assembly. However, I'm currently in the late stages of finishing another book (unrelated to DocBook or computers), and that prevents me from starting on the fifth edition. I would expect to be free in 6-9 months to start on it, and then several months to complete it, since it now has become a major update.
Bob Stayton
Sagehill Enterprises
bobs@sagehill.net