I recently purchased the WS4777 (3-way wall switch module) and an extra slave switch to replace an existing 4-way. I removed all the old switches and replaced the one's with 3 wires without a hitch. The big question is how to I connect the extra slave (3 wires) to the existing 4 wires (2 red; 2 black) in the box? Did I somehow order the wrong switch?

See the schematics at [Link: ourworld.compuserve.com]

The extra (second) slave switch would be wired just like the first.

Thanks for the link, but I'm still confused. The second slave only has 3 wires, but I have 4 wires in the electrical box. Which wires do I connect?

OK, I've updated the page with a diagram showing a 4-way installation. I'm sure it'll be obvious once you see in drawn out.

Just use the same link from above.

It worked! Thanks for your help.

I connected the 2 red wires from the box with the red on the slave and did the same with the 2 black wires and one of the blue wires from the switch. I put a wirenut on the other blue wire and I was in business. Now all three of my switches work.

Now my question is . . . how does it work. The original middle switch had a red and black on one side and a red and black on the other side of the switch. I understand how it works with switching the legs. Does the slave send a signal back to the master switch?

Yes, the slave is sending a signal back to the master and the master is controlling the load. That's why each slave is connected to power and to the extra wire which goes back to the extra wire on the master. In my scematic, the red wires from the Romex are used to carry that signal back to the master.