Britbrat - "As with just about anything, there are lots of compromises to be made in engine choices."

That pretty much sums it up.

I really don't have anything significant to add to the previously posted general personalities of 4S vs 2S. However, one should note that these are very general, and many engines do not fit well into their class description. For example, a YS 4S. True to 4S nature, they produce very good power at low RPM, but unlike many other 4S engines, they are fuel hogs and are not cheap to run. Also, the lines between 2S and 4S tend to blur as displacement increases. For example, a YS160DZ 4S is very similar in performance and response to a 1.60 2S OS with pipe. Both turn about the same prop at the same RPM, etc.

Cheers.