Windows Small Business Server 2011 Essentials Update Rollup 4 has been released today and is available for download from Microsoft Update. You can read about the issues this rollup addresses in Microsoft Support Knowledgebase 2757007.
In addition to the server-side update, a client-side update has been released as well. This update is documented in KB 2757014. The client-side package will be downloaded to the server along with the Update rollup. User does not have to go to the Windows Update website to obtain the package separately for the client machine. The client-side update supports Windows XP SP2 and later.