best thing is to do a plug chop. run your moped at full throttle for a few minutes, then kill the engine. take out the spark plug and look at it. it should be a nice golden brown (or tan at least). if it's gunky, you have too much oil and will eventually foul out your spark plug.
too much oil will also cause carbon build-up in the exhaust port and on the piston head. too little oil, on the other hand, can cause your engin to seize.
i use a bit more than 2.5 oz to the gallon, closer to 3oz. most people i know use about 2.8-3.0oz. 4 oz however is too much, i think.