SQL Server 2008 SP1 states that Cumulative Updates 1 to 3 are included in it. For those customers who have CU4 already installed, you need to know how it affects you. Have a look on the CSS blog: http://blogs.msdn.com/psssql/archive/2009/04/09/sql-server-2008-sp1-and-cumulative-updates-explained.aspx

CU 1 for SQL Server 2008 SP1 will cover all the fixes included in SQL Server 2008 RTM CU 4. CU1 for SQL Server 2008 SP1 was just released and is available http://support.microsoft.com/kb/969099

[Edit 2009-04-16 10:00. Added Link to CU1]