It depends on how your network is setup, if you have a router chances are that you'll need a software on that fifth computer.
Try ezProxy, it has bandwidth control if you have a ICS (Internet Connection Sharing) setup or you can install it on the fifth computer and remotely manage it from your computer.
I believe it's fully functional for 30days after that it's free but you can only have a limited number of groups. You'll need groups to be able to control the bandwidth.
You can find it at
http://www.lavasoftware.net/en/conte...y/features.htm