Actually, just put one line in 4wd rego forms:
"Warning: if another motorist or pedestrian is found to have suffered a fatality due your driving a 4wd without a professional reason you will be held liable for manslaughter"
That ought to scare a few clayfield mums off. Anyway, if you kill someone through negligence or inappropriate use of equipment with anything else it's manslaughter, so why not 4wds?
Imagine I'm licensed to own and use a rifle (or I don't even need a license). I'm using the rifle to open bottles in my front yard when I miss and kill a pedestrian walking past. The law would rule that I can't say it was just a freak accident since it's irresponsible to use a rifle to open a bottle.
Yet someone can use a 2 ton truck to go to the hairdresser, lose control on the way back and plough into a compact car killing the occupants and walk away without penalty.