I am replacing an old non-programmable with a Honeywell RTH230B. The old thermostat used four wires (out of 5). There are two main wire groups... one has a red a black and a white, the 2nd group has red and white only. There are 5 terminals on the thermostat...
Rh Rc W Y G, I can't figure the corresponding R, W, and Blk. Thanks a million.

Can you tell me how the wires were conndcted to the old thermostat.

Also can you look at the wiring at the furnace and tell me how the wires are connected there.

Here is what I think:
The Red should connect to the Rh
The White should connect to the W
The Black should connect to the G
These are that group of wires that were three together.

The group of two wires, one of them should connect to the Y.

Now if they are connected to the circuit board in that same fashion that is good, then one wire should be going from the C terminal on the board to the Air conditioner along with a wire that is doubled with the wire on the Y.

Rc gets connected to Rh which gets connected to R. Now you have 4 wires. The same as before.

If you really won't to figure it out you can test the combinations of the 4 wires. From there, I can figure out what everything is. The states will be nothing, fan comes on, heater+fan comes on and outdoor unit comes on. In this case you can safely touch the wires together.

Easiest way is to look at the terminal block on your furnaces circuit board. The wires will terminate there and be lettered. Match these up with the corresponding terminals on your new stat.