Not a licensing guru at all, but according to this, https://support.microsoft.com/en-us/lifecycle#gp/gp_lifecycle_servicepacksupport:
When support for a product ends, support of all the service packs for that product will also end. The product’s support lifecycle supersedes the service pack support policy
Of course, since SQL Server 2008 SP4 was released AFTER the end of mainstream support for SQL Server 2008 you would think it would be an exception, which is supported by information found here, http://blogs.msdn.com/b/sqlreleaseservices/archive/2014/05/29/update-on-the-service-pack-plans-for-sql-server-2008-and-sql-server-2008-r2.aspx:
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Will the lifecycle dates for SQL Server 2008 and SQL Server 2008 R2 change as these Service Packs are releases?
Yes. As an exception, the Service Pack Support end date of SQL Server 2008 Service Pack 3 and SQL Server 2008 R2 Service Pack 2 will be extended by 12 months to facilitate the upgrade projects of our customers. The actual lifecycle dates will be refreshed on the lifecycle page of SQL Server 2008 and SQL Server 2008 R2 as SQL Server 2008 Service Pack 4 and SQL Server 2008 R2 Service Pack 3 will ship.
SO based on this, https://support.microsoft.com/en-us/lifecycle/search?sort=PN&alpha=Microsoft%20SQL%20Server%202008&Filter=FilterNO:
Microsoft SQL Server 2008 Service Pack 310/6/2011Review NoteReview Note10/13/2015For IA64 only, SP3 will be supported until 7/9/2019. For more information see the SQL Release Services blog at http://blogs.msdn.com/b/sqlreleaseservices/archive/2014/09/30/sql-server-2008-service-pack-4-has-released.aspx
Microsoft SQL Server 2008 Service Pack 49/30/2014Review NoteReview NoteSupport ends 12 months after the next service pack releases or at the end of the product's support lifecycle, whichever comes first. For more information, please see the service pack policy at http://support.microsoft.com/lifecycle/#ServicePackSupport.%5B/quote%5D
Which says that the service pack support end data for SP3 is 10/15/2015 then you are going to get at least that long for SP4. I'd contact MS Support services to be sure though.
Jack Corbett
Consultant - Straight Path Solutions
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