I recently purchased Honeywell programmable

I recently purchased Honeywell programmable thermostats to replace Rodgers-White mercury thermostats. I somewhat understand the universal color codes for heating/cooling (R,G,W, Y, R (Rc,Rh) and C) but of course mine has to be different. I have a 2 wire system (no A/C) gas fired boiler, hot water baseboard. The baseplate on the mercury thermostat has the red wire connected to R (also labeled ''5'') and the white wire connected to a terminal labeled only with ''4''. The G, W, Y and C termials are not used. For a 24 volt 2 wire system shouldn't it be R to R (supply 24-volt power) and then to W for heat? What do I connect the ''4'' wire to on the new Honeywell? I assume the ''W''?? Help.