Hello
After installing SP4 you can take a full backup of C drive, System state & shadow copy components of your backup exec server.
Its recommended to update the remote agent after installing any service pack.
To update the remote agent open BE console, click on Tools->Install Agents and Media server on a different server.
Follow the wizard to update all the remote agents.
Refer to
http://support.veritas.com/docs/313285
Note: Before pushing out updates remove all the .msp files from the udpates folder which are older than SP4.
Just keep RAWS322213RSP4.msp & genpatch.out file in the udpates folder
The updates folder is located at X:\Program Files\Symantec\Backup Exec\Agents\RAWS32 & RAWSX64.