You can share, but you're not going to be doing it using the "router" part of the LinkSys box.
If I had to take an educated guess, I'd say that the LinkSys is a five port switch (which they already make) and two port router. One port of the router is hard-wired into one of the switch ports (hence, only four usable ports on the switch, instead of the five that are normally available).
To do modem sharing, you'll be using just the switch part of the LinkSys. The switching part will work regardless of whatever layer 3 protocol is being used, or what layer 3 subnet is being used (you can run multiple IP subnets over the same physical switch).