SBS 4.5 Service packs

I have SBS 4.5 which was installed a year ago on our compaq proliant server.
It runs IIS, proxy, sql and exchange.
I need to update it to the latest service packs and fixes, but I cant seem to find a service pack for the SBS 4.5.
Can i go ahead and install the NT SP6a, or do I have to get something special for the sbs.
I was not able to find any answers on the microsoft site. Please help.