We just moved and had to change plugs on the dryer from 3 prongs to 4.
What is the fourth prong for? I know about positive, negative and ground, but I can't think of why a fourth would be needed.
In the United States, most residential electrical service is what's known as a split-phase system. Which is a 3 wire, single phase system. The service entrance cable consists of two ungrounded (hot) conductors, and one grounded (neutral) conductor. Once at your house the grounded (neutral) conductor is bonded to earth, and a fourth conductor is added to the system. The fourth conductor is the equipment grounding; or simply grounding, conductor.
But why 4 wires?
A split phase system is aptly named, because the transformers secondary is center tapped (or split). This allows the system to provide different voltages, depending on how devices are connected.
If a device only requires 120 volts it will be supplied using one grounded (neutral) conductor, either one of the ungrounded (hot) conductors, and one equipment grounding conductor.
If a device requires 240 volts it will be supplied with two ungrounded (hot) conductors, and one equipment grounding conductor. In this case a grounded (neutral) conductor is not required, since the circuit is completed simply by connecting a load between the two ungrounded conductors.
Some devices require both 120 volts, and 240 volts. These devices will be supplied with both ungrounded (hot) conductors, a grounded (neutral) conductor, and an equipment grounding conductor.
But wait... The old one only had 3 prongs?
In the olden days, we didn't care as much about safety (there were fewer lawyers around). Before houses were wired with the equipment grounding conductor, devices that required both 120 volts and 240 volts would be supplied with both ungrounded (hot) conductors and a grounded (neutral) conductor. Since there was no equipment grounding conductor, the circuit lacked one. This is why older homes only have 3 prong dryer (and stove) receptacles. The old three prong receptacles are NEMA 10 devices, and consist of two ungrounded (hot) conductors and one grounded (neutral) conductor (hot, hot, neutral).
The US electrical distribution to the standard home is split phase AC - two hot wires, 180 degrees out of phase with each other with respect to the third wire, called a neutral wire. You can get 240VAC from the two hot wires, or 120VAC from one hot wire and the neutral wire.
Dryers typically require 240VAC, so they need, at minimum, connections to the two hot wires. However, some dryers also have components, such as timers, lights, and other accessories, that only need 120V. So they also include the neutral to the dryer.
In order to ensure user safety and reduce the chance of electrical shock, all user accessible metal parts of appliances must be grounded. This provides a shield of sorts - if the wiring, motor, or some other part inside the appliance breaks, and touches another metal part of the appliance, then the user could get a shock. If the metal is grounded, though, then the current is shunted to ground, and should trip the breaker due to too much current. Even if the breaker doesn't trip, the voltage on the metal will be very low due to the grounding of the appliance.
The neutral wire is connected to ground at the electrical box. Up until a decade ago, or so, it was ok for 240V appliances to pretend the neutral was also a ground, and use the neutral to provide safety for the appliance's metal parts.
However, it really isn't very safe, and the updated code now calls for a real ground to be attached to the metal parts of the appliance.
So the current specification calls for two hot wires, one neutral, and one ground.
When is a real ground different than neutral? The neutral wire can be carrying current. If the internal timer connects to hot #1, and neutral to get 120V, then the hot wire supplies the current and the neutral wire returns it (simplified, but essentially true). However, the neutral wire has resistance. It might be very low resistance, but when you push current along that wire, the voltage on the end closer to the dryer goes up slightly. This puts an AC voltage on the neutral wire at the dryer, which then appears on the metal parts of the dryer. It's a very small voltage in normal operation, but it's deemed safer to keep that separate from the user, and have a real ground on the dryer chassis. During abnormal operation, an unsafe voltage can appear on the neutral wire. So a ground wire is now standard.
The way that 240 power works is that you get two lines of 120 into the outlet with a common neutral.
The four prongs are two hots, a neutral and a ground.