Andy, our 42 inch Shetland can tow the 4 wheeler and 2 adults and 2 kids.He can do 10 miles with a few hills and suchlike.
The rule of thumb is 2 and a half times their own body weight, or combined if more than one. IF you live in East Anglia, more.
Welsh A's can work with 2 wheelers and two adults of average size all day.
Don't forget big wheels, once rolling will take the vehicle along and the pony is hardly pulling at all once you are on the move. So long as you have the 2 wheeler balanced well, there will be no weight on the horse's back at all.