This is the second post through pages of comments 1 outright stating 4x4 gives 4 wheel braking and this post implying that error.
since braking is a mechanical function on the tractor, you can not gain front brakes by being in 4x4. It is a physical impossibility! If your tractor does a nose dive, you will not have brakes! You can stop the rear wheels/tires from spinning, but has they have no traction, you have no brakes for slowing the tractor!
Now in fairness to this discussion, being in 4x4 if you lose rear traction does allow you to have some speed control as opposed to free spinning front wheels/tires as in 2x4 mode. No brakes but speed control/driven traction. What this may allow is more reaction time to deal will a potentially dangerous situation.
If you have HST, get it to slower speed of travel with the pedal. Do not ho to the neutral setting of the pedal. Do not go to reverse as you probably will loose traction and limited control. If you have a loader, making an assumption that you do with a nose dive, drop the loader to the ground (ok it is on the ground with the nose dive, still drop it, it should reduce the nose dive, bring the rear to contact the ground and provide some brake function.
Now if you have figured it yet, you need ballast or more ballast to avoid this nose dive. While filled tires is one type of ballast, you need weight behind the rear axle to offset the weight in front of the front axle. Your mfg should provide this info in your tractor manual or your loader manual.