Why can only 4 computers have access to my wireless network?

We have a wireless network which in theory should be able to allow more than 200 pc’s on the network. Now when we hook up a fifth computer, one of the other 4 gets knocked off the internet.

The setup is as follows.

Cables come out of the wall, go to a Telenet Network Interface Unit (Telenet is the provider) This in turn goes to 2 black Motorola boxes (no idea what they do there) and then on to a Lynksis WRT54G wireless router.

Any idea what the problem could be?

Thanks