I have never checked my wired unit but i would think all four would be used. Two for power to the iss and two for the signal path.
That's correct. In the standard configuration, four wires.
If external power is used for the wired ISS - or when there is more than one console - then the wiring will be a bit different.
Yes, the two inner wires are for signal, the two outer wires are for power.
If you add a second console, only connect it to the two inner wires.
My rain sensor has been separated from the temp/hum sensor and some of the elevations have changed, but this diagram shows some wiring details http://www.flickr.com/photos/30112397@N06/2879504922/in/set-72157607366155684/