|"Is this where building a forest or trust comes into play?"|
Depends. How many Domains are you planning on setting up. If only one then you don't need to worry about it.
This may help...
"i know i can set up the other 3 as workstations"
Yep that's how you do it. It ads and OU to the Active Directory just as it does for a workstation. What makes it a server and not just a Station is the roles you install on it. If you install File Server Roles then it becomes a server. I think you are reading too much into it.
As for a simple help guide on networking you might want to get a book because there is a lot to networking beyond servers.