I took the old thermostat off, and had G,Y,W,Rc and '4' terminals connected to the Green, Yellow, White, Red and Black wires respectively.
Voltage from Rc to G|Y|W was 24VAC as well '4' to G|Y|W was 24VAC
all other combos read less than one volt, so I'll assume that's just noise.
The new Thermostat requires GYW, Rc and Rh (or a wire to Rc and a jumper to Rh)
So, what's the Black wire for? Do I just tape it off and ignore it?