This service pack fixes several major bugs present in 1.0.1.SP2 and previous releases.
The most serious bug is http://jira.jboss.org/jira/browse/JBMESSAGING-721, which was causing message redelivery not to function properly. This bug fix alone should justify an upgrade to 1.0.1.SP4, especially if you use Message Driven Beans and Container Managed Transactions.
Another critical bug is http://jira.jboss.org/jira/browse/JBMESSAGING-787, which was causing long-running invocations (such as sending a large transaction to the server) to time out inadvertently. The root cause was identified to be http://jira.jboss.org/jira/browse/JBREM-691. The new Remoting library incorporated by 1.0.1.SP4 contains the fix for this bug.
Other bugs fixes address the following 1.0.1.SP2 problems:
Broken QueueBrowser behavior (http://jira.jboss.org/jira/browse/JBMESSAGING-803)
Non-compliant client ID behavior - the previous version allowed simultaneous clients with same client ID (http://jira.jboss.org/jira/browse/JBMESSAGING-791)
Incorrect handling of the replyTo property for persistent messages in some situations (http://jira.jboss.org/jira/browse/JBMESSAGING-757)
Corruption of the security context in managed environment (http://jira.jboss.org/jira/browse/JBMESSAGING-807)
Potential deadlock during the client initialization (http://jira.jboss.org/jira/browse/JBMESSAGING-800)
Memory leaks (http://jira.jboss.org/jira/browse/JBMESSAGING-638)