NT Server-Do I Really Need User Licenses

Our company has a small peer-to-peer network using a server running NT 4.0 workstation for file sharing and minimal security. We are also running Client Access for an AS400. We want to begin using VPN with the internet, and upgrading to NT Server seems the most cost effective way to go. What does a User License do for each workstation? Do I need them to run the network as I do now? Do I need them to run VPN? Do I really need a license for each and every workstation? Several questions, but all intertwined. Thanks for your responses.