Microsoft Dynamics CRM Rollup 5 (v4.0 R5) has been released can be downloaded here.
"The build number of the update rollup packages for the Microsoft Dynamics CRM 4.0 server, for the Microsoft Dynamics CRM 4.0 client for Microsoft Office Outlook, and for the Microsoft Dynamics CRM 4.0 E-mail Router is 4.00.7333.1644."
Key items within this rollup include tweeks to performance such as fixes for the following
956527 (http://support.microsoft.com/kb/956527/ ) The Microsoft Dynamics CRM client for Outlook consumes three times as much memory in version 4.0 as in version 3.0
959072 (http://support.microsoft.com/kb/959072/ ) An entity form loads slowly after you add many fields of several different attribute types in Microsoft Dynamics CRM 4.0