BackOffice Small Business Server 4.5 has been released by Microsoft, and a free upgrade is offered to registered SBS 4.0 licenses. Details are at Microsoft. Please hold off on the upgrade until ESG gets a chance to do some testing. Someone from that group will post a follow up on the test results when they are complete.
We will continue to support SBS 4.0 for the forseeable future so there is not technical reason to upgrade yet. I expect Microsoft will discontinue SBS 4.0 shortly however, so new installs with use 4.5.