When wiring a 4-way switch circuit, What were doing is simply controlling the power flow (Switching off/on) to the load (a light, lamp, outlet, ceiling fan etc..) from more than 2 different locations. a couple examples would be:
At each end of a hallway and a switch halfway down the hallway. or at the top & bottom of a stairway and a switch by the front door.
We will assume that the circuit in these examples are controlling the power source to the same load. (a Single Light)
Before going any further with the 4-way switch instructions, I would recommend you go to my Wiring a 3-way switch page and first become familiar with that circuit and how it functions.
OK, assuming you are all caught up on the 3-way concept, we will continue by making a 3-way circuit into a 4-way circuit. All that is required is wiring a 4-way switch between the (2) 3-way switches. Now don't let this be confused with where the switches are physically located in your home. This is how the circuit is structured. The switches can be physically located to your liking but you must keep in mind the ease of routing the wiring from point to point. Also, this example is just adding a single 4-way switch which will give you 3 locations to control your light. To add more locations to this circuit you just simply add additional 4-way switches in series with the existing 4-way switch.
The diagram below will give you a good understanding how this circuit is wired.
As you can see, this is basically a 3-way circuit with a 4-way switch added between the (2) 3-way switches. Again, if you choose to have more locations controlling the load, then add additional 4-way switches. Keep in mind that the 4-way switches must always be between the (2) 3-way switches.
Now lets look at it from a flow point of view:
Using your mouse pointer, follow along with me as we go through the circuit. Now this one gets a little tricky by keeping track of what switch you toggle in your mind, just hang in there.
Now the power source (black) is coming in from the left. and goes through the 3-way to the black wire going to the 4-way. Then through the 4-way to the red wire going to the 3-way. Now it dead ends (not connected) which means the light is turned off. By toggling any switch, the light will now turn on. Choose a switch and follow the flow of the switch that changes state. Keep in mind how the 4-way switch works. Left connections toggle between to two right connections. So top left red connects to bottom right black and when toggled top left red connects to top right red. The bottom pole works in the same manner. It goes straight across.
Allow Tim Carter from AskTheBuilder.com to show you some great tips for installation and wiring a 4-way switch.