What is there to ballast on a 4wd? I am glad you asked.
Proper ballasting of a 2wd tractor for tillage work would include filled tires and cast iron rim weights. Basically, more weight yields more traction.
The rear wheels of a 4wd tractor should do just as much work as the 2wd version, therefore filled tires and iron weights. The front axle is meant to be front axle assist. It is meant to provide additional traction, not to be a substitute for the rear axle doing the work.
The fully weighted tractor can be too heavy for yard work, and more than a person wants to haul behind their truck. And then there is the money thing.
Both 2wd and 4wd need rear ballast, as well as suitcase weights or loader for front end weight. Shannon can add $1500+ to the quotes for proper ballasting.
The farm is getting smaller.