In theory, you may only need two wires! ON a four-wire plug, you have two hots, neutral and a safety ground. On a 3-wire, you usually (I think) eliminate the safety ground and neutral and internal ground a tied together in the device. The neutral is required on some devices to provide both the 120 vac (for say for the electronic controls or lightbulbs). All newer devices are required to include the safety ground, so it needs to be a 4-wire cable.