The RU4 for Exchange Server 2007 SP1 was released, as follows:
You can get it here: http://www.microsoft.com/downloads/details.aspx?FamilyId=8B492ED2-EA92-412F-A852-3AA1C58D9499&displaylang=en
Description of the Rollup Update 4 for SP1: http://support.microsoft.com/?kbid=952580