Todd, you are mixing some terminology here.
Forest is a collection of domains that share a common schema. A tree is a
collection domains with contiguous namespace (a.local, b.a.local,
c.b.a.local,.....). A site is a group of well connection networks. You
create sites, so that you can control active directory replication. Site
basically describes a physical network topology to AD meanwhile
forest/trees/domains define logical structure of your network.
With that said, based on your description, you could create 4 sites, for
your office locations. The decision on multiple domains/trees is based on
your security requirements and company structure. Normally people go with
one Forest/domain, and then organize network resources in OU's. You can
create different OU's for your office locations and the below OU create OU's
to further group your resources (computers, users, departments,.....).
--
Regards
Matjaz Ladava, MCSE, MCSA, MCT, MVP
Microsoft MVP - Active Directory
(e-mail address removed), (e-mail address removed)
http://ladava.com