How to Install a 4 Way Switch
Before you can install a four-way you need to understand how a three-way circuit works so please see my how to install a three-way switch before attempting a four-way. Also please keep in mind that I only show one way to install a 3-way switch. There are few ways to wire a 3 way.
In order to have a four your need to have a three-way. No matter how many switches you want in your circuit you will only have two three way switches. You’ll have one at the beginning and at the end. All switches in between will be a 4-way switch. Even if you intend to have only three switch’s or ten of them.
So let’s say you have an existing three-way circuit and you want to add another switch to it. All you have to do is find the (3-wire) running from one 3-way switch to the other 3-way switch and install the 4-way switch between the two. Now that’s sounds easy but you will most likely find that wire way to short to just cut it and install it in a new switch box.
So in short, you would cut the wire in between the two 3 ways and install a 4-way switch. This is meant as a guide. Your wiring could be very different.
No matter what you can never ever use a ground wire for anything else but a ground.