Ask The Hardware Guy website has released a video on how to wire a four way switch.
Feb. 12, 2013 - PRLog -- A 4 way switch is used when you have three or more locations that operate a light or group of lights. The lever does not have an ON or OFF label. A 4 way switch is always wired between a pair of 3 way switches. When you have more than 3 switches, you will always have a pair of 3 way switches and all the other switches will be 4 way switches. The 4 way switches will always be wired between the pair of 3 way switches.
A 4 way switch has 4 screw terminals and a green grounding screw. The 4 screw terminals will be grouped in pairs. They may be marked Line 1 and Line 2, or Input and Output, or you may have two screw terminals colored dark and two screws colored light.
To replace a 4 way switch, you will first turn off the power to the circuit that you are working on. After unscrewing the cover plate and the switch, pull out the old switch and mark one group of wires with masking tape. You want to identify the pair of wires going to Line1 or Input or the dark colored screws. These 2 wires will be coming from one cable that is grouped together. In the box, you will have 2 cables connecting to the 4 way switch. You may or may not have a grounding wire.
The pair of wires from one cable should be identified before you remove the old switch. If you removed the old switch and did not mark the wires, you need to find the 2 cables. Each cable will have 2 wires or 2 wires and a ground, grouped together coming into the box. These 2 wires must be connected to one of the paired groups of screw terminals.