Release 18.0.4 contains a number of bug fixes and enhancements, including a fix for a deadlock that can be triggered by using the topology UI, and a rather large performance fix for startup if you are using BSM.
For a high-level overview of what’s changed in OpenNMS 18, see What’s New in OpenNMS 18.
For a complete list of changes in 18.0.4, see the release notes.