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 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 switches 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 sounds easy but you will most likely find that wire way too 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.