From the Microsoft website:
Warning: Microsoft has found an issue with the final build of SP4 that
impacts customers who run SQL Server with Address Windowing Extensions (AWE)
support enabled. This issue only impacts computers with more than two gigabytes
(2 GB) of memory where AWE has been explicitly enabled. If you have this
configuration, you should not install SP4. Microsoft is currently working on the
problem and will issue an update soon.