Download: Rollup 4 for Exchange Server 2010 Service Pack 1
Click here to download Rollup 4 for Exchange Server 2010 Service Pack 1.
Some of the known issues were addressed @ Update Rollup 4 for Exchange Server 2010 SP1:
2465292 "MAPI_E_FAILONEPROVIDER (0x8004011D)" error message when you access an Exchange Server 2010 mailbox by using a MAPI application
2394554 An email message is not delivered if it contains unsupported encoded characters in the subject line in an Exchange Server 2010 environment
2491951 You cannot install Exchange Server 2010 SP1 if the NetBIOS domain name of the domain controller contains an ampersand (&) character
2500648 "There are no items to show in this view." error message when you try to view a folder in Outlook in an Exchange Server 2010 environment
2533538 You cannot look up the free/busy information of a user who is located on an Exchange Server 2010 organization from another Exchange Server 2010 organization
2506820 The free/busy information does not display of a user whose mailbox is located on an Exchange Server 2003 server
You can get the list of issues fixed in this Rollup available here.