Edit: Update Rollup 18 is now available for Exchange 2010 SP3 see the post Exchange 2010 SP3 Update Rollup 18 installation tips.
Less than three weeks after Microsoft released the Exchange 2010 SP3 Update Rollup 3, they have now released Update Rollup 4. Update Rollup 4 for Exchange Server 2010 SP3 contains the fix for the security issue that is described in MS13-105. This is the only difference between the two Update Rollups.
The update can be downloaded from here, and the kb article 2905616 can be found here.