They build small tractors with four wheel drive because two wheel drive is not sufficient to keep us from getting stuck with what we put our tractors through. Four wheel drive CUTs were first developed in Japan to use in rice paddies because similiar sized 2 wheel tractors (like Ford 8Ns) wouldn't steer in that environment. Since I come from mud land, I always have the 4X engaged. The differential lock is an afterthought.
Follow these four steps when using your tractor off a roadway: 1) Always use four wheel drive on rough ground, particularly when using an implement, or on hills, or if you have to be in low gear just to make the terrain. You have more pulling force in 4x, or put another, way less wasted horsepower since more of it is engaged with the ground. 2) If you have a hesitation, raise the implement. 3) if that doesn't work to get you going then stop, engage the diferrential lock and try again. Be sure you can afford to go only STRAIGHT forward or backward, otherwise try the brake trick alluded to previously. 4) If still stuck try to back out using the FEL to push you. If after that you're still stuck, then consider a winch, wrecker, Cat, a new tractor, or logs chained to the back rims, in that order.
Finally DO NOT be afraid that you will wear out your four wheel/front wheel drive. I will last a s long as your tractor does - usually longer than you will keep it anyway. (That said, turn off the 4X on roads and in the barn cleaning stalls to save front tire wear.)
Mf