If the circulation pump located between the buffer tank and the heat pump unit only runs when the heat pump runs, where is the temp probe located with in the system that activates the unit when the temperature drops in the buffer tank?

The desuperheaters I know of only run when the heat or cool cycle is running.  It has no dependancy on the temperature in the tank.  The buffer tank is only intended to shield or "buffer" your final hot water tank from the full load of incoming cold water.

Ed 

For the buffer tank on the DHW Ed is right.

For the storage tank on the radiant system,  you install an aquastat that senses the temp in the tank and turns on the heat pump when needed.   You want the largest tank you can afford for the radiant storage tank to give you longer run times.

If larger is better, what is an optimun size for a 4ton unit on the radiant system

The cutoff seems to be at 119 gallons. If you go over that then the prices jump a lot.

80 gallon would also be an option for a 4 ton unit but I would not go smaller than that.

My 3 ton FHP HP has an 80 gal buffer tank. The temperature  bulb, an RTD, is clamped (& insulated) to the line between the tank and the circ pump. The pump then pushes water through the heat pump and into the top of the tank.

To smooth out the operation I added a seal-in relay across the HP thermostat terminals. Once the thermostat calls for heat the HP will run until the Temperature switch is satisfied independent of whatever the load side / zone pumps call for. It also makes a fan start delay unnecessary as the warm water is circulated to the air handler immediately.

FWIW I believe there is a requirement for ASME certification on pressurized tanks 120 gallons and up. That drives up costs and explains why tanks of the odd number of 119 gals are available.

A Marathon 105 gallon tank is another, though pricey, option