I believe in the release note for 4.0, it does mention of the file.
tc5-cluster-service.xml is needed in addition to the existing cluster-service.xml to configure the http session replication. This is because since 3.2.6 and up, the Tomcat http session replication has been implemented using JBossCache.
I have added this question to the cluster faq Wiki, nonetheless.
We are also in the process of updating the Admin book on this part (due out for 4.0.2).