The JBoss folks have written excellent docs on clustering. It explains what clustering in jboss means, the mechanisms and technologies they use to pull it off. How to configure it. And they even dive into some code.
Do yourself a huge favor and get the JBoss Clustering docs
This wiki is also helpful as it covers a few topics from the docs in greater detail:
The PDF link you provided does not exist.
I found 3 PDF files here:
The most recent of these is dated 5/21/2004.
(Jboss AS 4.0.0 was released 9/20/2004.)
So will there be an update to this doc covering Jboss 4?
A new version of the Clustering guide won't be produced any time soon. However, the old version is quite accurate and pretty complete, even for JBoss 4.
There is a clustering section in the JBoss Application Server Guide as well; the most recent version of that was produced in I believe October.