As far as your question about the 4 foot elevation difference, correct, the elevation difference between the inlet and outlet can be 0� and it will still siphon. Shouldn�t matter. What matters is the outlet to pond waterline elevation difference.
Not totally sure why the vent tube has to go 2� into the air either. May be just a personal preference thing or there might be something more technical to it. We usually just come up a few inches with the vent tube and add angled fittings until the vent is running parallel with the slope of the inlet pipe. Then we install a perforatied sewage pipe to function as a vent tube trash guard and use big stainless hose clamps to clamp the vent tube to the 6 or 8 inch inlet pipe.
Also, the �normal pool� will be the elevation of the vent tube inlet. Once the thing goes into siphon after a rain event, it will stop flowing at the vent opening elevation. I would call that normal pool. Again personal preference. If it flows but doesn�t go into siphon, then the pool elevation would be at the point A on the drawing.
Having said that, that�s a good drawing, and if you build it just like it depicts, it should function correctly.
