The four wires are:
-taillight (running lights)
-L turn
-stop (they used to be seperate)
-outbound light to the center light
To hook up the modern two wire light:
-taillight to taillight
-L turn to the other wire
-stop - cap-off
-outbound wire still goes to the to the center light
A test light makes all this easy,
Best of luck,
RJ