I will try to help clarify this. If your car has coil packs that fire the plugs instead of a distributor or coil on plug/near plug system each spark plug fires once every revolution of the crankshaft instead of once every 2 revolutions so since they fire twice as often they last about half as long. Each coil in a coil pack ignition system fires 2 plugs with one firing tip positive and the other tip negative (half the cylinders fire positive and half negative) In an inline 4 engine with a standard 1-3-4-2 firing order plugs 1 and 4 share a coil and 2 and 3 share a coil. When 1 is firing at the end of the compression stroke 4 is firing at the end of the exhaust stroke and vise versa with the same true for 2 and 3. Since there isn't suposed to be anything to burn during the exhaust stroke the spark is wasted which is why it is called a waste spark system.