In the tank mix 3:1 water to Optimum Carwash (or any other you prefer), then set the slider to the 3rd hole from the left (4:1). This combo will give you 1 oz per gallon at the car.
I am not sure I understand your ratio. If in the tank you mix three parts water to one part Opt Carwash that would mean the mixture is 25% carwash or 32 ounces per gallon. If the foam gun is set 4:1, then I assume the foam is mixing four parts of water for each part of solution from the tank. Woudn't that come out to about 6.4 ounces (32 divided by 5) of wash to a gallon of water?
Please don't take this to mean I am arguing with you, just trying to understand the ratios.