Update Rollup 21 is available for Microsoft Dynamics CRM 4.0. It can be downloaded from the Microsoft Download Center or Windows Update as of February 9, 2012. This update is available for all languages supported by Microsoft Dynamics CRM 4.0 and includes several improvements and critical fixes. The following components are affected

Components:
Microsoft Dynamics CRM 4.0 Server
Microsoft Dynamics CRM 4.0 E-mail Router
Microsoft Dynamics CRM 4.0 Language Pack
Microsoft Dynamics CRM 4.0 Reporting Extensions
Microsoft Dynamics CRM 4.0 Report Authoring Extension
Microsoft Dynamics CRM 4.0 for Microsoft Office Outlook

Supported Operating Systems: Windows 7, Windows Server 2003, Windows Server 2008, Windows Vista, Windows XP

Prerequisites: You must have Microsoft Dynamics CRM 4.0 installed to apply this update rollup. Additionally, Update Rollup 7 for Microsoft Dynamic CRM 4.0 is required to install update Rollup 21. This update requires Microsoft .NET Framework 3.5.

Important Note: Update Rollup 21 for Microsoft Dynamics CRM 4.0 cannot be removed. Specific database updates are applied in Update Rollup 21 that will not let you uninstall Update Rollup 21. You should back up your databases before you install this update.

Links:
Microsoft Support: Update Rollup 21 for Microsoft Dynamics CRM 4.0 is available
Microsoft Download Center: Update Rollup 21 for Microsoft Dynamics CRM 4.0