Wiring a three-way switch circuit is one of the more complex electrical jobs that a homeowner can attempt. And adding a four-way switch to the mix can make an already difficult job even more confusing. But if you don't rush through it and you pay attention to the directions, wiring a four-way switch into a three-way circuit will be easier than you might think.
Turn off the circuit breaker that powers the circuit to which you will be wiring the four-way switch. Use the voltage tester to ensure that the circuit is OFF.
Look at the four-way switch and notice that it has four screw terminals plus one ground screw. The other two three-way switches in the circuit have three terminals and a ground screw.
There is a length of NM cable (ROMEX®) that travels between the two three-way switches. This line needs to be split at the point where you are installing the four-way switch. If the walls are already finished with drywall, it will be very difficult finding this cable. This job is preferably done in the roughing-in phase of remodeling, however, if you take your time and don't rush things, you should be able to track the cable down.
Cut the cable in half and remove about 6 inches of sheathing from each side using the ROMEX® stripper. Then, remove about 3/4 inch of insulation from the ends of each wire. There should be a red, black, white and ground entering and leaving the four-way switch box once you're finished.
Connect both white wires together with a wire connector and stuff them back inside the wall box. Take a spare 6-inch length of ground wire and connect it with the two ground wires in the box so you create a pigtail. Connect the pigtail to the ground wire on the four-way switch.
Take the black and red wires that are coming from three-way switch No. 1 and connect the black wire to the top screw on the left side of the switch and the red wire to the bottom screw on the left side of the switch.
Take the black and red wires that are going to three-way switch No. 2 and connect the black wire to the top screw on the right side of the switch and the red wire to the bottom screw on the right side of the switch.
Wrap the switch's terminals with electrical tape and secure it to the wall box. Install the switch plate, turn the power back on and test your four-way switch installation.