This update contains a number of customer-reported and internally found issues since the release of RU3. For a list of changes included in this rollup, see KBA 2509911 Description of Update Rollup 4 for Exchange Server 2007 Service Pack 3. We'd like to specifically call out the following fixes which are included in this release:
2531208 You cannot synchronize a folder hierarchy by using Outlook for Mac 2011 in an Exchange Server 2007 SP3 environment
2528437 EWS applications cannot connect to Exchange Server 2007 servers after you make changes on accepted domains
2521063 You are incorrectly displayed as a meeting organizer after you synchronize the meeting by using your mobile device in an Exchange Server 2007 environment
Update Rollup 5 for Exchange Server 2007 Service Pack 3 is currently scheduled to release in August 2011.