Four Way Switch
The four way switch is used in between two three-way switches to control an outlet or light fixture from three different locations. If you wanted to control from more than three locations, say five locations, you would still use two three-way switches (one on each end) but now you'd use three four-way switches in between the two three-ways.
The four way switch looks the same as a double pole switch except that a four-way switch will have no "on" or "off" markings. The four-way switch has four terminals most always brass in color. There is no COM or "common" wire like a three-way switch. The four-way switch simply functions as a switching device for the traveler wires between the three-way switches.