Not all the small 4-strokes are quiet or trouble free - I bought a new 4hp Yamaha 4-stroke last year for my 10ft Sniper dory and it's a dog of an engine. Incredibly noisy for its size and due to a design flaw in the positioning of the carburettor (which I finally got Yamaha to own up to at the Boat Show), takes up to 11 pulls of the recoil starter to fire the bitch up.
Yamaha UK told me the way round the problem was to fit an external fuel tank and use the bulb to pump the fuel through, thereby negating the convenience of having a portable engine
Its other little foible occurs when you tilt it up after it's been running. The hot sump oil somehow runs past the piston rings and soaks the spark plug so when you come to start it again, it won't! All in all a vile piece of equipment, and due to Yamaha's initial denial of anything problematical with the engine, led me to disregard them when specifying an engine for my Avon Searider. Furthermore, mine isn't a one off - I know two people with the same engine and both have the same problems.
I would also concur with the above views regarding transporting it in the car - it really is a dreadful bore having to worry which side to lay it down on and in hindsight I would have definitely gone for a 2-stroke.