How to Install a Four Way Switch

Installing a four way switch to control a light fixture from more than two locations is a bit complicated. It's not recommended to do this project yourself unless you're very familiar with electrical matters. Here's an overview of how to install a four way switch in a hallway between two 3 way switches. The first three way switch is already wired to the electric box.

Four way switch
Black, white and ground insulated wire (wire A)
Red, black, white and ground insulated wire (wire B)

Directions

Shut off the electricity at the panel box. Each connection between the switches requires a separate length of wire.
At the four way switch location Run a length of wire B from the four way switch to the first three way switch. Run another length of wire B from the four way switch to the second three way switch.
From the three way switch to light fitting Run a length of wire A from the second three way switch to the light fixture.
At the first three way switch Connect the white wires from the three way switch and the four way switch using a wire nut.
From the first three way switch to the four way switch Connect the black wire from the first three way switch to the side of the four way switch. Connect a black wire on the other side of the four way switch to the second three way switch.
From the first three way switch to the four way switch Connect the first three way switch to the four way switch with the red wire.
From the four way switch to the second three way switch Connect the red wire from the four way switch to the second three way switch.
At the four way switch Wire nut the white wires together at the four way switch, with 6 inches (15.24 centimeters) of white wire. Secure the wire to the ground screw.
At the second three way switch Connect the white wires with a wire nut. Ground the wires as above.
At the light fitment Wire nut the black wires together and the white wires together [source: Wake Forest University, Do It Yourself].