There are about a million ways to wire up a 3 way switch, which makes it about 10 million ways to wire up a 4 way switch... There is plenty of online information about using a single slave switch with the standard X10 3 way wall switch and while I found articles that claimed it was possible to have multiple slave switches they fell a bit short in actually explaining how to do it.

As per ususal when reading this page keep in mind that I'm talking about lethal things. If you're not secure working with this stuff, don't know how to switch off your circuit breakers or are otherwise unsure of yourself, don't do this. It can kill you immediately and kill your family later when it burns down your house.

Turns out that the slave switch is nothing but a pushbutton switch, there is no circuitry or anything exciting inside. The 2 blue wires are just connected together, and when you push the button they connect to the red wire. This means that you should be able to chain as many of them together as you like. In reality the longer run and more switches you have the more noise and resistance you'll get on those other wires and the harder it will be for the switch to tell when you're pushing the button and at some point you'll start to have problems. I have done two 4 way circuits at the house now and so far they are working fine.

I was lucky in my house as the load is at one end of the chain and the feed is at the other. There are so many complicated and strange ways to wire it with the load in the middle, or the feed in the middle somewhere that I can't cover them here. These pictures will only help if you're lucky enough to have it layed out in a straight line. (at least wired in a straight line, it doesn't matter physically where the switches are.)

Wire up the switch and the last slave switch as documented by X10:

Not too easy to see in that picture, but 1 blue and the red to the 2 travellers and the other blue to the load. You can wire them backwards with the switch at the load side and the slave at the feed side, but some people say they have problems with this.

Then, for as many switches as you need in the middle wire like this:

Basically the 2 travellers should be connected together straight through the box. The slave switch has BOTH blue wires connected together to one traveller, and the red wire to the other traveller.

And if you have enabled local dim on the switch in question, it will work from all the slave switches.