There are two distinct motors out there that most people mistake as "gas." Glow, and true gas.
For your application (80 inch wingspan), I would recommend you go with a gas engine. This will burn a mixture of gas and oil, just like a weed eater engine would. In fact, lots of people (myself included) have converted weed eater engines for use on airplanes.
You can get a large glow engine, but the cost of glow fuel coupled with the consumption of said fuel in these large glow engines make them very expensive.
Saito does have a four stroke gas engine that has valves and spark ignition. Don't know much about it, other than it is expensive. It would still have to have oil mixed in the fuel, just like any other two stroke. Only the delivery of the fuel is different.
On the other four stroke glow engines, yes, they have valves just like a car. You can usually tell the four strokes from the two strokes becuase they will have external pushrod tubes, and usually a rocker arm cap on top of the jug. That, and they sound awesome.
However, they burn glow fuel, not gasoline, and they do not use spark ignition. Instead, they use a glow plug that keeps the cylinder hot between combustions, and the fuel/air ignites off of compression temperaturesmuch like a diesel engine.
Hope this helps.