Yeah, unfortunately there's no WAN port on the Shaw router. From the Shaw router's perspective, "WAN" is the cablevision line coming in.
I'm told that in Bridge mode, you connect your own router to port 1, but thinking about how it must be wired internally, i'd bet that the four ports are just a dumb switch, and if the Bridge mode is enabled that just pushes DHCP to the next level up, ie. at Shaw. So you could use all four ports if you could get Shaw to issue four IP addresses, but they couldn't talk to each other through an "internal" network.
I have already ordered a second 50' Cat6, and a 4-port Ethernet switch. It means a second cable running down the baseboard between the cabinet under the TV where the Shaw modem will live, and the closet under the stairs where the Asus router will live, but I can live with that:
Shaw Modem (Bridged)
Asus Router (gets external IP from Shaw, serves DHCP internally)
| +-- Three ports for my office
One port back out to the room with the Shaw router
| +-- Rainforest
+-- WDTV Set top box (or another nVidia Shield TV in the future, maybe... best TV box ever...)