Exchange 2013 Service Pack 1 which is essentially CU4 but is named SP1 instead. it contains some new functionality such more DLP functionality and the big one, support for Windows Server 2012 R2 Domain Controllers, raising both Domain Function and Forest Functional Level to 2012R2, and installing Exchange 2013 SP1 on Windows 2012 R2.
Other new stuff is that Edge server is back. A new protocol used for client/server communication is introduced called MAPI/HTTP which is very similar to MAPI over RPC which is then tunneled over HTTP so in short, communication don’t rely on RPC layer which gives some advantages when it comes to authentication and reconnection of clients. It is disabled by default and you also need Outlook 2013 SP1 for leverage MAPI/HTTP.
If you use TMG or any other reverseproxy that filter on URL’s you need to add ‘/mapi/*’ as an allowed URL.
As usual there is the regular schema and AD prep stuff to do before you install the first SP1 server.