I would strongly suggest that SP4 be installed on the local machine at the
console during a maintenance window with nobody logged into Terminal
Services. You should reboot immediately after applying SP4. Also, make
sure to archive your previous configuration during the process. If you have
to rollback be aware that you'll need to re-enter any passwords associated
with scheduled tasks.
After applying SP4 you may want to have the fix referred to in MS KB Q324446
applied if you start to experience slow logins or sessions not being
released.
http://support.microsoft.com/default.aspx?scid=kb;en-us;324446
If you literally can't be on site, then I would download SP4 to the machine
in a TS session and then walk someone who is onsite through the install.
SP4 changes the way that the SIDs are stored on the local system. That
means that no new sessions will be able to log onto the server once the SP
is installed until the system is rebooted. So, if you lose your session
during the process you may not be able to reconnect to the box to finish the
process.