First, let's get our definitions right . The term 4x4 means four-wheel-drive, and SUV means sports utility vehicle. A Fiat Panda 4x4 or an Audi A4 quattro both have transmissions that send power to four driven wheels, but they're not the sort of full-size SUVs that people so often object to.
As long as we're clear about that, we can go ahead and discuss the damage the most polluting cars do to the environment and how they should be dealt with. They're not all SUVs. Even an "economical" diesel Range Rover emits 299g of carbon dioxide per km, while a supercharged petrol one pours out 376g/km. If you think these are scandalous, how about a Ferrari 575 Superamerica at 499g/km or a Lamborghini Murcielago at 500g/km?
However, while the most consumptive cars of all are the ultra-fast and fairly rare supercars, most of the vehicles that would attract new, higher taxes are the heavy, gas-guzzling and far more common 4x4s that also happen to be the targets of opprobrium from those who find the things an affront to sensibilities of green-ness, efficiency and social conscience.
Good thing too, I say. As someone who likes cars and intelligent engineering solutions, I fail completely to see the point of a hefty 4x4 unless you need to tow a heavy trailer over tricky terrain. I hate the dimwitted notion that they are somehow safer than normal cars. Safer for whom? Certainly not the occupants of a car they might hit. I hate the way the high headlights dazzle drivers in lower cars. And I hate the way the 4x4s' excess width makes passing impossible in lanes which once flowed freely.
And there's that whole arrogant, selfish mindset that must accompany every unnecessary purchase of a fat 4x4. I'll park where I like, they seem to say, I'll block the road and you'll just have to wait. They are large, over-engineered and full of unnecessary lumps of transmission. This makes them thirsty, and their weight accelerates the disintegration of the roads that the tax-disc was originally meant to pay for. And they are both tall and bluff, so they have to shift a lot of air as they move which makes them thirstier still.
But if they are so thirsty, are their drivers not paying extra tax already with each guzzled gallon? Yes, but the deterrence factor appears to be minimal. You could argue, then, that a small rise in road tax will be similarly ineffectual, and you would probably be right. Good try, though.