We are four developers in different locations (in a 100 km radius of each other) tryint o collaborate on a software development project.
We would like to install Tea
TFS is just a bunch of web services and should be set up for remote access out of the box.
Things you'll probably need to configure.
Make sure that the windows firewall is allowing TFS to accept incoming requests on port 8080 (the TFS install may do this for you)
Configure your router to use "Port Forwarding" so that requests from the internet to port 8080 are routed to the machine with TFS installed.
Your ISP probably allocates IP addresses dynamically so you might need to sign up for a Dynamic DNS service such as NO-IP.com. (check which ones are supported by your router)
Once you've done this then you should be up and running.
I'm sure others will suggest that you ditch TFS and use a DVCS such as GIT or Mercurial, they have a point! You should consider if it's worth the effort of getting TFS to work in this way when another system might be easier to get up and running.