Based on the MS wording in this KB article (KB2958696), it doesn't look like there will be an SP4 for 2008.
This article describes cumulative update package 17 (Build number: 10.00.5861.00) for Microsoft SQL Server 2008 Service Pack 3 (SP3) . This update contains hotfixes for issues that were fixed after the release of SQL Server 2008 SP3.
Note This is the last cumulative update for SQL Server 2008. Mainstream support for SQL Server 2008 ends on July 8, 2014. For more information about the lifecycle for SQL Server, check out the Microsoft product support life cycle information.
Anyone planning on staying with 2008 for a while may want to consider applying the CU since there has been 17 of them.
17 CU's and no final service pack. :crazy: Unbelievable!
I added my vote to the count.