Tractors are either 4WD or 2WD. Most modern tractors are 4WD as they are reliable and especially useful on slippery surfaces because they provide more traction than 2WD tractors.
2WD tractors were more popular than 4WDs in the past, but 4WDs are now more popular. A 4WD is recommended when driving on hilly terrain because most offer better descent control than 2WDs. They use engine braking in conjunction with the front and back wheels, whereas 2WDs engine brake only with the rear wheels. Uphill driving is also safer with a 4WD because their weight is distributed towards the front end.