Two strokes can be very quiet. Use a good muffler, use vibration isolating soft mounts, and use larger or higher pitched props to keep the RPM down. I used to fly a .40 powered plane from the shores of the lake that I live close by. A neighbor actually mistook it for an electric plane because it was so quiet.

Four strokes can be quite loud if they aren't muffled well. If fact some of the loudest engines I have heard were four strokes. How loud can a four stroke engine be? To find out, I invite you to visit a NHRA sanctioned drag strip and stand close to the staging area as two nitro burning, supercharged rails drop the hammer as the christmas tree turns green. Back to model airplanes, if you really think the four strokes are so quiet, walk a mile away from the flying field and listen. You will still hear the four strokes after the sound of the two strokes has faded into the background.