The difference between a three-prong plug and a four-prong plug for a stove, dryer or other 240 volt electric appliance is the addition of a separate ground wire to improve consumer safety. On the three-prong plug, two of the prongs are the hot feed, each providing 120-volts of power, and the third is the ground-neutral wire.
According to About.com, the theory behind the three-prong plug is that electrical panel manufacturers bond the neutral to the ground bar in the box, giving both the same electrical potential. However, in wet locations, such as the kitchen or laundry, the human body becomes a path of least resistance through which electricity flows, increasing the potential for electrocution if there is a fault in the appliance.
In the event of an electrical short, a human touching the machine becomes the path to ground the power takes. If the person happens to be standing on a wet floor, the 240-volts of electricity passes through his body. Even if he survives the incident, he often suffers serious burns.
The four-prong setup includes two hot wires to provide power to the appliance and a neutral wire to return electricity to the circuit panel. The fourth wire is the ground. It provides a low resistance path for the electricity to return to ground in the event of a malfunction, increasing safety for the user.