Ah yes, 4 way switches...don't let them scare you. If you want to switch a light from two locations, you would install two 3way switches. If you want to switch from 3 locations, you would use two 3way switches and one 4way switch. Four locations, two 3ways and two 4ways, and on and on.

Now here's that basic 3way system.

In the diagram above, power comes into the switch box on the left, then a piece of 3 wire runs to the second switch. That piece of 3 wire carries the neutral, ground, and the two travelers. Imagine then, for a basic 4 way system, you want to add a switch box between the existing two and run that 3 wire from the 1st box, to the 4way box, to the last box and then a 2 wire up to the light. See the diagram below. Now we have added the box in the middle. At that box, the grounds are wirenutted together (of course, the box and device are grounded if needed). The neutrals are wirenutted together. And finally the black and red wires are hooked to the 4way. So if you think about it, you could continue to install 4ways between the 2 3ways and have as many switches as you want.

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