HoneyWell Wifi Thermostat and C wire
I just got off the phone with Honeywell.   If you only have a 4-wire older system, you'll only be able to use the fan in the "auto" position.   It won't let you use the fan-only selection.  You need a 5th wire for power for the wi-fi side of the thermostat.   Call Honeywell and they'll walk you through it for free.