Home Light Automation - Emulate 4 way switch with relay
You can purchase and utilize a common DPDT type relay (Double Pole / Double Throw) to create a cross over switch. It is pretty easy to understand from the following relay diagram:



You connect one circuit to the two NC (normally closed) terminals of the relay. The other circuit connects to the two COM (common) terminals. Then you add two additional wires which cross connect the two NC terminals to the opposite NO (normally open) terminals. 

The coil of course is driven with the control voltage that you have selected. The simplest control voltage to use would be +12V DC. There are plenty of relays available that have 12V DC coils and your remote automation control can use simple transistors to drive the relay coils. 

If you go ahead and use 24V AC for the relay coils then you would have to use an additional relay for each switch control to translate the switching from the AC realm to the low voltage DC realm. (There are circuit components available that can switch the 24VAC but it gets more complex than necessary for an application where +12V DC can be easily used instead. And then you can derive the needed +12V DC from a re-purposed computer power supply. 

A major concern when building any type of system such as this, where you bring remote control wiring into the mains wiring boxes of your house, is safety and isolation. Only relays rated properly should be used and you would bear the safety liability associated with stringing the control wires around your house.