I have 4 PC's at home. I want to setup a LAN connection between them all for file sharing via Ethernet cables, and no internet access is available or needed. I have successfully connected the first two with a simple cross-over cable, but I am lost on how to connect all 4. Because I won't use internet, I assumed that I won't need a modem either, am I wrong?

1. Should I use a modem/router?
2. How do I? Without internet, that is.