This is the second release candiate release of 4.0.
When upgrading from a previous release you are advised to clear down the journal files and persistent database first before upgrading to this jar as the binary format of messages is not compatible with 3.x or earlier 4.x releases.
The 4.x branch of ActiveMQ has now moved to the Apache Incubator so the package names have changed from org.activemq to org.apache.activemq. For a full list see the Changes in 4.0.
For a more detailed view of new features and bug fixes, see the release notes
jiraissues: Unable to determine if sort should be enabled.
Also see the previous ActiveMQ 4.0 M4 Release