With general use and maintenance 3 ml is fine. When dealing with decay already established in the mouth, we have made the solution a little stronger.
Using 3 ml gives you a concentration of around 800 PPM available chlorine dioxide.
Using 4 ml gives you a concentration of just over 1000 PPM available chlorine dioxide.
The goal is to be within the range of 600 - 1000 PPM, so either solution will work.
An excellent disinfecting spray is made by putting 2.5 ml of 22.4% sodium chlorite into a glass and adding 2.5 ml of 10% citric acid . Swirl to mix and let this activate for 10 minutes. Pour this into the spray bottle and add enough water to make a total of 500 ml.
Keep in mind that the 22.4% concentration of sodium chlorite can be dangerous to handle. Take the proper precautions when mixing up these solutions. A 5% concentration is much safer to handle so I recommend people dilute the 22.4% concentration down to 5% and use that.