Since 1996 new homes are required under Article 550.16(A)(2) of the National Electrical Code (NEC) 2008, the dryer outlet shall be installed with a 4 wire electrical supply so that the neutral and ground wires are separated and not carried on the same wire.
There are three terminals and a ground wire that make up the 4 wire connection. The two outer terminals are the hot terminals, and the middle terminal is the neutral terminal. So if it is a 3 or 4 wire cord does not matter, the dryer terminals will always be the same.
It is possible to install a 4 wire cord on pre-1996 model dryers, for this you connect the white(neutral) wire from the dryer cord to the middle(neutral) terminal on the dryer. Then connect the green wire to the grounding screw located on the frame of the dryer. The black(hot) and red(hot) wires from the cord will get connected to the two outer(hot) terminals on the dryer.
Before installing a 4 Wire dryer cord on any dryer, you must first remove the connection made at the factory between the neutral terminal and the ground(which is the metal frame of the dryer). It will either be a copper strap that is connecting the middle(neutral) terminal to the metal frame of the dryer or in some cases it will be a white(or green) wire connecting the neutral terminal and the grounding screw(located on the frame of the dryer).
If your dryer has the copper strap, remove the strap from the neutral terminal this will not be reconnected when upgrading to a 4 wire cord. To break this connection if your dryer is equiped with a wire connecting the neutral treminal to the grounding screw simply unhook the wire from the ground screw and connect it at the neutral(middle) terminal with the white wire from the new 4 wire cord to prevent it from contacting any other terminals or the frame of the dryer.
If you dont remove the grounding strap(or wire) from the neutral terminal when upgrading from a 3 wire dryer cord to a 4 wire cord, electric current can be carryed on through the grounding strap(or wire) to the frame of the dryer. This will create a shock when you touch the outside of the dryer.
If your dryer has a existing 4 wire cord and you are replacing the old dryer cord with a new one, you will not have to worry about disconnecting the factory connection between the neutral terminal and the ground. Rewire the new dryer cord wires in the same locations that the old wires were removed from.
Important!!! Make sure you unplugged the dryer cord from the outlet before removing the old cord from the dryer.