If you are experienced enough or have staff that is experienced in server administration then I would definitely recommend going with a dedicated server as a client/server model. If you don't have alot of know how when dealing with servers and things like domains, DNS, DHCP, groups, security permissions, etc. then a peer-peer network may be the way to go and just buy a standard work station.
Even with only four PC's, I personally find a client-server model much more easier to administer and control access to files and services (such as printer) versus peer-peer. And not to mention you can shut down any of the workstations and not have to worry about files, drives, or printers being unavailable. With a dedicated server all resources are always available regardless of who turns off their PC.
I run a dedicated server that I built on my home network which runs Win2000 Server and love it! I will never go back to a peer-peer model. With having over 10GB of MP3's and music videos it makes it really nice to block songs with obscenities from my kid's access. And not to mention limit their access to the laser printer when it seems their finger is glued to the print button. Sure I could do the same with P2P but with the server it just seems easier and I don't have to screw with adding the same user accounts to all computers and stuff like that. All administration is centralized.
Just my opinion... I'm sure other's will disagree since your only networking four computer. By principle you only need a P2P model but for added efficiency and management, a client-server model is the way to go. So buying a server would be a good thing. Plus as someone else had mentioned you will already have the infrastructure if you ever decide to add more computers or place an additional load on your network.