Four

Anybody in a region can "see" objects and AV's in the directly bordering region. To accomplish this, a fair amount of communications is needed between the servers involved. So if the neighboring region send out a lot of updates, this might impact the region you are in.

In general, a physical server will host more than one region at a time. Although this is normally totally invisible to the users (and even the owner), a high load on other region can impact the performance of the region your are currently in.