Don't refer to AC wires as negative or positive; they're hot and neutral. Confusing me 
The neutral/blue out of the white transformer cable is split into four and one goes into each panel -- the remaining pad.
All of the relay boards discussed thus far should be fine for your use. The one you have linked from Cool Components is a bit mysterious on the 10-pin block; I'm not familiar with .NET Gadgeteer cables. It's fundamentally identical to the one I linked to at Arduino Direct but I'd have to see a closeup to tell which pin is which.