I puchased a programmable T-Stat and there is no wiring diagram. It suggests that I label the wires attached to my existing t-stat connected to Yi/Y2, G, RH, C etc.; however, my existing stat is an older hard wired mercury switch type with no labeling. I have 4 wires coming from the handler - Blue, red, green & white. Can someone shed some light? Its getting hot in here.

Go to the furnace end of your tstat wires. Trace each colored wire and tell us what its going to,, if you look very close I can`t imagine a thermostat with 4 wires to it not labeled, sometimes the post/lables are hard to see,, also do you have heat only,, or heat and a/c

With only 4 wires coming to the stat, it's easy to identify. Take any two wires and connect together and post a table.

... blue red green white
Blue.
Red.
Green
White.

Post as Excel

Obviously RED-RED shoud not be done and if you did BLUE-RED you don't have to do RED-BLUE.

The three events your looking for are:

1. Only the fan comes on
2. The heater and fan come on
3. The AC comes on.