The answer is in the description:
A TAG-(or LTAG)-equipped unit can make a special weapons attack in order to designate a target.
The only rules that allow you to "target" the ground are the base arty rules (POI), but that's not a special weapons attack, it's an arty attack. There are no rules for making a special weapons attack against the ground, therefore you can't TAG the ground.
Also, you wouldn't get the -4 for an immobile target against the ground - "immobile" is a unit movement ability status. The ground isn't a unit, therefore it can't be immobile.