As others have pointed out, the tractor has a "wet disc" brake inboard of each rear wheel. You can see the brake housings from the back of the tractor on the axle tubes. These brakes are mechanical, not hydraulically assisted like a car. (Things change when you get to really large tractors).
As DirtyDeed mentioned, even when 4x4 is engaged, only one of the front wheels is "guaranteed" to have braking effect. This is because braking power from the rear is transmitted through the front 4x4 driveshaft and to the front differential. Because these tractors have "open" front differentials (meaning not lockable or limited slip), power will always go to the path of least resistance. This is true when you are accelerating the tractor or braking the tractor. The path of least resistance always means the wheel with less traction, unfortunately. So if you are trying to make it up a hill with good traction on one side and mud on the other, in 4x4, the front wheel with all the power will be the one in the mud. Likewise, coming down the hill, the wheel with the braking action will be the one in the mud (in the front). This same thing applies to most all cars and trucks on the road. However, in many cases on a tractor, the ground conditions are similar under the wheels and power will apply roughly equally.
Last piece of advice. Use 4x4 for coming down slopes, but I wouldn't recommend leaving 4x4 engaged at all times. If nothing else it will greatly increase wear on your front tires and create a larger front turning radius. It will also cause binding on tight turns and add stress to your tractor's drivetrain.