[Jon T]

Sorry Jimbo, your going to need a new router.

The four ports on your existing router are on one side of the NAT(network address translation) process, and the ADSL feed from your phoneline is on the other. This is what danielf meant when he said that they were only output ports, yes ethernet ports are bidirectional, but in the case of a router only one port(and that can be an ethernet or ADSL port) can take the main IP feed and feed it into the NAT process in your router.

An ADSL router internally is lad out like this:

ADSL Modem <-> NAT/Firewall <-> Ethernet Switch(usually four port)

A cable router is laid out as like this:

Ethernet WAN Port <-> NAT/Firewall <-> Ethernet Switch((usually four port)

So the reason that your PC's can communicate with each other is that they have a direct path to each other via the switch part of the router.