Wikipedia describes four-stroking as:
Four-stroking is an undesirable operating condition of two-stroke
engines, where they instead begin to fire every four strokes, rather
than every two strokes. This firing is uneven, noisy and may even
damage the engine if allowed to continue unabated. Four-stroking was
often a cause of poor idling in two stroke engines.
It then describes what's happening:
Two stroke engines rely on effective scavenging in order to operate
correctly. This clears out the combustion exhaust gases from the
previous cycle and allows refilling with a clean mix of air and fuel.
If scavenging falters, the mixture of unburnable exhaust gas with the
new mixture may produce an overall charge that fails to ignite
correctly. Only when this charge is further diluted, by pumping
through a second volume of clean mixture, does it become inflammable
again. The engine thus begins to 'fire-and-miss' every second cycle
(every four strokes), rather than correctly on every cycle.
So, yes it's true that it is only igniting every other intended power stroke. In some worse cases an engine can six- or eight-stroke, igniting every third or fourth power stroke.
The reason seems to be that the exhaust isn't purged completely and leans out the mixture on the next stroke and isn't rich enough to ignite leaves a mixture on the next stroke that cannot be ignited until the following stroke which then produces exhaust that again isn't purged completely. The more exhaust left in the cylinder, the more skipped strokes.
Model engines are affected differently because of their small scale:
Four-stroking is a common and expected behaviour with model engines,
both glow fuel and diesel. These small engines run at extremely high
rotational speeds and their scavenging relies upon this. When started,
they run as inertially-scavenged four strokes and have a distinctive
change in engine note when they accelerate past the point at which
they begin to operate as two strokes. Owing to the scaling laws of
such small engines, this four-stroking is an unavoidable consequence
of limitations on their scavenging at slow speeds. However the same
scaling laws also make the effects of four-stroking less severe and so
the engines can idle happily in this mode, without damage.
I suppose that the glow-plug in these engines remains hot enough to ignite the fuel but, every other stroke is just too lean to ignite not an ignitable mixture. If it started six- or eight-stroking maybe it would cool too the point where it could no longer ignite the fuel on the strokes with an appropriately combustible mixture.
