Ok...The port Forwarding site doesn't have a link for the 4200, but it does for others I will give you the link and you will have to have a look and see which one looks the same as your router set-up..Start with the 5200 and then you will have another page pop up, scroll down to L and click on limewire...
Oh the reason is your router has more than likely got an inbuilt Nat Firewall....I have searched the internet for the exact instructions, but I have had no luck..I think most of the speedstreams should be similar...
PortForward.com - Free Help Setting up Your Router or Firewall