How do I properly wire a 5-wire thermostat? What is this ambiguous '4' label?

Question

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?

Answer 1

I found the manual for the old thermostat on the web.

Turns out '4' is an alternative label for Rh on some old models.

I guess I needed that.