You are correct. Z-Wave device ordinarily use RF only to communicate (Insteon uses the powerline as a means of communication. Insteon RF is a back-up and may not be as strong/dependable as Z-Wave, but I'm unable confirm that.) But, Z-Wave secondary switches are different and use a traveler wire, not RF, to communicate with the primary switch in both directions. That makes them somewhat less costly.

So does that mean a traveler switch wouldn't need a strong mesh network to function if it has the traveler wire to communicate? Similarly, I assume it would not contribute to the mesh network? Any RF capability?

Correct on both counts. It the slave/secondary switch was RF, then a traveler would not be required.

Stu, your solution worked.  I used one of the two Travelers to bring Line to the main switch and  the 4-way is doing its job. Thanks