The grounding wire on a kiln is fastened to the kiln switch box with a small green bolt. The connection must be tight, because the grounding wire prevents an electric shock should a current-carrying wire come off a terminal and touch the kiln's case or switch box.
4-wire kilns cause a lot of confusion with the grounding wire. 3-wire kilns are easier to understand. (The number of prongs on the plug indicate the number of wires in your system. But that can be deceptive, because people install 3-wire plugs on 4-wire kilns.)
With a 3-wire, 240 volt kiln, the green grounding wire does not carry current. It is attached to the kiln case. A 4-wire kiln has a neutral ground wire and also a grounding wire. The neutral ground wire carries current during normal operation. The grounding wire does not. It is for safety only.
Some people eliminate the grounding wire of a 4-wire system, thus rendering the kiln less safe.