As has probably been said here already (haven't read the whole thread) it's completely absurd to ban / penalise / tax / road charge vehicles based on the number of driven wheels. How do the "anti 4x4" brigade propose to define what is or isn't a 4x4? Is every Audio Quattro a 4x4? How about a Fiat Panda 4x4, or Subaru Impreza or Mitsubishi Evo? Is a 2wd Honda HRV a 4x4? What if, in reaction to 4x4s being penalised BMW et al made 2wd versions of the X5, ML Class, Touareg, RX300 etc? Would they (no longer 4x4s) suddenly be OK?
As Matt F has said, any special tax should be based on fuel consumption figures or size of footprint or some other non-arbitrary measure that has some environmental relevance.
Michael.