I use the herpstat 4, but that is because I have multiple cages...
for a single 40 gallon I would get the herpstat 1 (or 2 if you need to use a heat lamp as well)
I would get a UTH that covers about 1/3rd of the bottom of your tank. attach the thermostat probe to the UTH in-between the UTH and the bottom of the tank.
IF your room ever gets below 75 degrees you will need a heat lamp as well as UTH's do not change the air temperature in the cage. if this is the case as I said above I would get the herpstat 2 so the heat lamp only runs when needed to keep the air temperature in the cage correct (around 80 degrees is best) I would use an infrared bulb for this.