Hi Parafer,
I'm moving your question to the SEP forum from the Altiris Client Management Suite forum.

However, if you do have Altiris as well, you could setup a package that gets distributed to package servers at each site and the clients at that site download from there, which would update the pattern files.  Also if your network equipment supports QoS, you could "de-prioritize" the traffic to/from the SEP server to ensure that critical business traffic gets through.

You can install four different SEPM on four sites
the bandwith depends when you replicated between each of them
if no replication , then you can have them individually
the clients will communciate with manager when heart beat happens
here is the info about that. 

https://www-secure.symantec.com/connect/idea/bandwidth-throttling

You know, in your situation you might consider running 1 SEPM and use GUPs to distribute the updates.
My current setup is similar, we have three locations. I have the SEPM at the data center and setup a GUP in each location. The GUPs were setup on boxes that have Windows Server 2003 installed, so that there would not be a connection limit. Most of the bandwidth used is from update distribution, not policy updates.

The built in Sybase database should be fine in your case. The client limit is 5,000. I would make sure to make a backup of the DB and the keystore on a regular basis. 

Mike

Have a look in this article
Tips For Installing SEP In A Low Bandwidth Environment