I recently purchased a 4 port router and firewall. How do I configure it so I can play multiplayer with a friend? Which IP address should I use? The one from the machine or the one given to me from my ISP?
You should use the external IP. Since the packets will now be being sent to the router rather than your machine you need to tell the router what to do with them when they arrive. Hence, you should configure "port forwarding/redirection/translation" to send all packets that arrive on port 4993 to your (internal) IP.