OK, basically a 4 stroke uses a pair of valves in the top of the head to control when fuel/air mixture enters the engine, and when exhaust exits the engine whereas a two stroke uses ports on the side of the cylinder to do the same thing. Because of the different designs one engine will fire when the piston has travelled down and up once, and the other will fire when it has travelled down and up twice (hence the names).

From a practical perspective as a modeller using a glow engine the main differences are:

- 4 strokes are quieter, and have a somewhat more "realistic" sound (as much as a 4 stroke mower sounds more like a plane than a 2 stroke one does). Many people find the 4 stroke sound more pleasant, but there'll always be those who prefer the 2 stroke sound.

- 4 strokes have a lower maximum RPM but more power, meaning that you can use a larger prop which will turn slower giving the same overall performance. This can be useful for reducing noise (the tips of the prop make a lot more noise as they approach the speed of sound), and can allow you to use 3 or more blade props that an "equivalent" 2 stroke won't allow.

- 4 strokes produce less power given the same displacement (size) engine... that is a you can't replace a .40 2 stroke with a .40 4 stroke, you'd generally want about a .52 4 stroke to replace a .40 2 stroke. You can generalise it as "for a 4 stroke, go to the next size or two up". This is because 4 strokes only fire half as often, but produce more torque when they do fire due to more efficient air-fuel/exhaust exchange. Many model manufacturers today will specify engine size ranges for both 2 and 4 stroke engines, but there's still a lot (mainly older designs) that only specify 2 stroke sizes and leave the rest to you.

- 4 strokes are more fuel efficient than 2 strokes... however because you generally have to use a larger 4 stroke engine this cancels out to some degree, leaving you with a similar flight time for the same tank size.

- 4 strokes are more mechanically complex, giving more potential for problems and requiring more maintenance. They not only have the two valves that a 2 stroke doesn't, but also a cam shaft & pushrods to drive the valves. They also have a breather tube and an intake manifold that 2 strokes don't. All this means that for similar power 4 strokes are often a little heavier too.

- 4 strokes generally have the carburettor mounted behind the cylinder on a manifold, whereas most 2 strokes have it in front of the cylinder mounted hard up against the crank case. This can move the needle further away from the prop making it safer to tune, but can present problems on models that have limited cowl length as it makes the overall engine longer. It's generally something you can work around but it's good to be aware of it. Try to read the manual and find out what depth the plane needs from the firewall to the prop driver then look at the engine's dimensions before you order a 4 stroke.

- The exhaust on a 4 stroke runs much hotter making it easier if you want to add smoke to your airplane.

- 4 strokes generally use a slightly different (and usually more expensive) glow plug.

- 4 strokes are more expensive to purchase (because of greater mechanical complexity).

- 4 strokes have smaller, more discrete mufflers (because they're quieter).

- 4 strokes generally run cleaner, leaving less mess to wipe off your plane/chopper/boat/car/hovercraft/ornithopter/mars rover etc.

- I've heard some people say that 4 strokes are more difficult to tune than 2 strokes. I don't personally feel this way, but some people find it more difficult to hear when a 4 stroke engine is "peaking" as they adjust the mixture. If you do find it difficult an optical tacho will help.

- I personally find 4 strokes more reliable with smoother idles and transitions, and seem to me to be less likely to "dead stick" for no apparent reason. Your mileage may vary.

- As far as I know, all multi-cylinder glow engines are 4 strokes. There's no reason you can't have a 2 stroke multi-cylinder engine, I just haven't seen one. This could be because multi's are so expensive to buy that most folks would expect a 4 stroke to help justify the cost?

- There is an exception to some of this information. A company called RCV Engines makes 4 stroke engines with a radically different design. They have 3 sizes in their SP series that have the cylinder "in line"... that is the cylinder forms the front of the crank case, and have a single rotary valve that takes the place of the two traditional "poppet" valves. These 4 stroke engines allow you to (for example) fit a 4 stroke engine into a slim warbird cowl without having to cut holes for a cylinder to poke out of. They also incorporate a gearbox which slows the prop down even more (at the same time increasing torque) allowing even larger props that are much closer to the correct size for scale. They are (of course) more expensive again than a "traditional" 4 stroke engine.

I use both 2 and 4 strokes... my 4's are on my scale models and my 2's are on my "sports" models. I don't plan on buying any more 2 strokes, but I also have no plans to replace those I have with 4 strokes.

Hope that helps some...