I won't dispute your fuel economy statements. But would argue you can't generalize about 4-stroke noise -- My 2012 Yamaha 20HP 4-stroke is neither quieter nor smoother than my 2001 Nissan 2-stroke 9.8. They're about equal in noise at idle, but at cruise speeds the Nissan is quieter AND smoother (less vibration). I used to own a 4-stroke 2001 Yamaha 9.9 that was also about equal in noise to the new 20, and same comparison -- definitely louder at higher speeds.

The Nissan is an exceptional engine, I'll admit, but it is a 2-stroke, 2-cylinder. It even idles smoothly without the usual 2-stroke "skip". It weighs 58 pounds according to the spec sheet. Show me a 9.9 4-stroke that comes within 20 pounds of that. Not even close.

I'll take 20-30 pounds lighter weight over a 40% fuel economy hit any day, in a dinghy engine. Fuel economy is more important on a main engine, but portability rules (in a dinghy).