You are lucky to have 4 feet. I have 8 inches or less around my house.
Zero behind the garage.

Is this going to be an antenna or just a tower for an antenna? Is the
area irrigated/watered for grass? If an antenna, you will need to
install a bunch of radials on or just under the ground surface. If you
are trying to ground the tower for lightening protection, I guess I
would mix a bunch of granular commercial fertilizer in with the soil as
I packed it around the ground rods. Then lots of water on the area.

Did you also have holes placed for guy wire anchors?

Paul, KD7HB