Gas and Oil Home Heating Furnaces - Changing to programable thermostat

I currently have an old thermostat and would like to switch to a programmable one (heat only). I removed the old thermostat to see what I had to deal with before I bought a new one. My old one has a clock wired directly (no batteries). Therefore there are 4 wires going to the thermostat. It is not clear which wires I need to 'cap off' (not connect to anything) and which 2 wire are for the thermostat. Am I assuming correctly?...that I need to only use 2 of the wires? If so, how can I tell which 2 to use? Thanks in advance.

Mike

interesting....2 of the wires make the heat and the other 2 power the clock. look at the labels were the wires attach to the t-atat. r and w are heat the other 2 are power to the clock