Noise is directly proportional to how much you lean on the throttle. Throttling back once airborne makes a big difference in the noise footprint. Next you can load the engine with a larger prop. The 4 strokes dont mind lugging a bit (doesn't work to well if you are doing aerobatics, but it you are just shooting circuits or flying around, it will help ease the noise). An after muffler will help...some 4 strokes have quite a bark to them which can be annoying to non-flyers. You are pretty much on your own here, but I have seen guys use that Aerotrend blue silicone pipe and strap on a lawnmower muffler behind the stock muffler. Another idea is to take an ordinary 2 stroke muffler, turn the baffle around and used that same silicone pipe to join the outlet of your 4 stroke to the outlet of the 2 stroke muffler. The exhaust travels backwards through the 2 stroke muffler and exits out of what normally be the manifold end. Beauty with this is that it can be mounted on the belly and the "outlet" rotated to point straight down....which keeps tthe gunk off your model... Dubro use to market the ARISE muffler (DUB707), but it has been discontinued, but still may be found in some hobby shops...it was a bolt on quiet muffler for 4 strokes.

One other tip...make sure that everything is tight on the model so there are no rattles, and balance the prop to minimize airframe vibration...if the model is ultra- or mono-cote covered, it can act like a drum if there is any resonance......