Normally the SP's Veritas makes are cumulative. The problem with SP4 or SP4a is that Veritas does not seem to be able to make this service pack to install correctly. It will tell you that you need to install SP2/3, or whatever, but that does not seem necessary. If you search this forum for 'SP4' you will find quit a few people had ran into problems while installing SP4(a) and a workaround on how to install SP4. (if you really want to)
If you, after reading those horror stories, still want to install SP4(a), do yourself a favour and make an image/backup of your BE server so you can restore it if installing SP4(a) f***** your system.
Regards
A van der Boom