The issue with thinking in terms of normal ground temperature is two fold. First heat pumps are removing energy in the winter so the ground might normally be 4C but it drops to say 0C. Second your working fluid never reaches soil temperature, so it might end up at -4C next to the pump.
Anyway, you only receive benifits when air temperatures are below the temperature of your ground loop. If the outside temperature needs to hit -4C before that happens you’re only benefiting from a smaller percentage of the year. Further that benefit is again constrained by the delta between resistance heating and what your ground source is providing the heat pump.
Generally it takes a combination of lower cooling costs in the summer and lower warming costs in the winter to make ground source heat pumps worth it, so being limited to a few months in the winter vs a 20+k installation cost is prohibitive.
Anyway, you only receive benifits when air temperatures are below the temperature of your ground loop. If the outside temperature needs to hit -4C before that happens you’re only benefiting from a smaller percentage of the year. Further that benefit is again constrained by the delta between resistance heating and what your ground source is providing the heat pump.
Generally it takes a combination of lower cooling costs in the summer and lower warming costs in the winter to make ground source heat pumps worth it, so being limited to a few months in the winter vs a 20+k installation cost is prohibitive.