I thought I had read somewhere on here that you could mix "ordinary" switches with PCS switches.

Anyhow, the wire from the 2nd switch box to the light (I'm assuming that's where the wire goes) is 12/3 w/grnd. The same coming into the 3rd switch box. After thinking about it some more, could wire #4 (in my wiring diagram) be the one going to the light? That seems to make a little more sense - explaining the purpose of that wire in that box.