SQL Server 2005 upgrade to Service Pack 4 (SP4) via command line
Make sure you include the /IAcceptSQLServerLicenseTerms switch, otherwise your installation will fail.  I have commonly done silent installs of service packs to using the following syntax:

.\&lt;SQLServicePack&gt;.exe /q /action=patch /instancename="FOOBAR" /IAcceptSQLServerLicenseTerms


Full syntax documented here.