Geotermal

It depends on where you are.

The temperature difference is negligible for use in geothermal power generation in all but the most volcanically active areas (which are again not suitable for equipment and personnel except under the most extreme of circumstances).

You can check for yourself if you have access to a spade and thermometer.

0.014285714285714285714285714285714° increase per foot of depth on adverage

"the temperature of earth in 4 feet depth" is 42°F, somewhere.

Where are you?

"...somewhere...Where are you?"...sir, you have a gift! :-)

http://www.duanesworld.net/duanesworld.net.sensors.htm

this link will help you...

If it is of much difference 4ft down compared to the surface, then one is in considerable peril.

I used to measure this in a job so long ago that it seems like it happened in another life. In the Northern Pa area, towards the western half the state, where the sky is cloudy 66% of the days of the year and the winter temperatures fall into the single digits, it is around 41 to 43°F. In the Austin Texas area where sunny days are vastly predominant and I doubt they ever see single digit temperatures, it is around 67°F. I barely remember W.Va. or southern Ohio, but think they were in the low fifties F.

Depends on the amount of solar heating an area gets on average and average mean temperature.