You will not and cannot solve societal ills by more revenue generating laws. We already have too many laws, most of them being far too verbose leading to confusion, and many are obscure, yet we can be and are often fined on the technicalities.
For your wish you would need to categorise exactly what is and is not a 4WD for the purpose of your law. License plate recognition would require coordination with (eg) Vicroads data. Are you aware a Toyota Aurion is a large car but a Camry is a medium car? Both have the same body. Is a Renault Captur or a Mitsuibishi ASX a 4WD, or only a Pajero or Landcruiser size for your purposes? A Renault Koleos, a medium SUV, is listed as a Car/Truck body type -> Wagon.
Since you are going to charge based on park size, let the Hyundai I20 or Fiat 500 use the 4WD park and pay for it. Right?
Be careful what you wish for. You might get it in unexpected ways.