When I first learned about how a four stroke engine works it actually blew my mind. Alphonse beau de Rochas first patented the engine in 1861 though years late Nicolaus Otto, A famous engineer came up with the concept and built the first car model that used the enging after reading a book on two stroke combustable engines while working at a grocery store in germany. I guess the book got his "Gears Turning".

The first thing you need to know is that there are four stages (Strokes) that make a four stroke engine run. Inside the engine block, there is a crank shaft. As the crank shaft turns it causes a lifter to make each piston move up and down. At the end of the crank shaft there is a cam shaft which is connected to the crank shaft by a timing belt, or a timing chain (Depending on the model. while the crank shaft is making the pistons move up and down, the cam shaft is turning making the valves to open and close. It's kinda like teamwork

On the first stroke the piston sinks down into the cylinder while gas is injected (on fuel injection models) and oxygen is pulled in through the air filter to mix into the cylinder. As you know, Oxygen will not ignite with out fuel, and fuel will not ignite without oxygen, so there has to be a good mix of the two. On the Second stroke the oxygen intake valve will close and the piston will ascend up the cylinder compressing the oxygen/gas mixture. This is called a compression stroke. 

On the third stroke after the Oxy/fuel mixture is compressed in the chamber, the spark plug ignites the mixture causing an explosion powerful enough to launch a potato 500ft. The piston gets blown back down the cylinder, The fourth stroke is simply releasing the exhaust. On this stroke while the piston is re-ascending the cylinder a valve opens to release the exhaust from the explosion.

The exhaust exits the engine block through a manifold, through a catalytic converter (Filtering the Polution) and down the muffler it goes. Now go out there and fire up your car and just picture it. It will blow your mind. and as for the four pistons on a 4 cylinder, or 6 on a 6, 8 on an 8 so on so forth, they all just take turns on the four strokes. the cool thing is, it's running on its own because of the turn taking it's doing, when one ignites and descends the cylinder, it causes the crank to turn another piston to another position and continues the cycle. pure awesome!