mod-cluster main page specifies JBoss AS 5 as a minimum requirement for mod_cluster.
But at the same time on mod-cluster documentation page we can read how to configure JBoss AS 4.3
Is it some kind of documentation mistake? Anyone runs JBoss AS 4.3 with mod_cluster on it?
JBoss AS 5 is the oldest JBoss AS with which one could get mod_cluster working without any additional development effort.
mod_cluster is added as a *.sar; suggested version is the latest in mod_cluster 1.0.x branch.
Lemme add that using software as old as this is discouraged; you should move to Wildfly and mod_cluster 1.3.x.