Three prong versus four prong dryer outlet.

The main difference between a 3-prong and 4-prong dryer is the wiring configuration:

3-prong

Has two hot wires and a neutral wire, but no separate ground wire. This configuration can cause an electric shock if the ground wire opens up. 3-prong outlets are more likely to result in shocks due to water from laundry.

4-prong

Has two hot wires, a neutral wire, and a ground wire. The separate return path for unused power eliminates the possibility of a ground current to the appliance.