Company installed a GMV95 gas furnace, using my old Honywell thermostat, single stage.
I believe that the 2-stage furnace needs a 2 stage thermostat. As of now the old thermostat is connected with 4 wires. If I upgrade the thermostat to a Honeywell visionpro RTH8500, won't I need an addit. 2 wires for the 2nd stage heat and A/C? Y2 and W2?
The company says no, I need only 4 wires. Don't have to run new wiring.
I am confused and want the new furnace to run as anticipated.

Your system can be set up to use just a single stage thermostat, the logic in the furnace control board will control high and low fire from time, not time and temp swing.

Thank you T-Top.
Is it better to use the 2 stage thermostat? Will the system run more efficiently?

If it was me I would use the two stage thermostat so I could have a quicker response to load demand.(comfort level)

Go for the Honeywell 8000 series t-stat. There are a few different models so I would read up on them first.

Catalog (http://customer.honeywell.com/honeywell/UI/Pages/Catalog/ApplicationCategory.aspx?Catalog=Homes&Category=Thermostats_102&ChannelID=%7b2EB2F178-20ED-44E0-97FB-CCFB4218DD64%7d)