The Piaggio MP3 reverse trike scooter:
As you can se it has three wheels, not four, but that's not my point here.
What I'm talking about is that the two wheels in the front counts as one if the track width is below a certain limit. I
think that limit is 465mm or so in Europe.
I guess a single wheel is always one, no matter how fat and wide it is. Putting two wheels tight together on a common hub (like on heavy trucks) should also count as one. I would definitely prefere this before a single tyre and a fat single tyre before a thin one. The single rear wheel is probably what worries me most when i drive my little Heinkel in high speed. -If it blows it's probably goodnight...