The front and rear end are open differential, but 4wd is not.I'm sorry but this is not correct. The open differential, with uneven traction such as going down a grassy hill, will end up apply the braking force to the wheel with the least traction. This translates to no braking force at all since braking a wheel with no traction is obviously not helping you. The wheel with traction continues to freely rotate. And it will also reduce or totally eliminate any braking force on the mfwd drivetrain for front braking. The differential lock failing on uneven slippery terrain is a very serious failure that will easily lead to this kind of runaway. Compounded by the gear shift slipping into neutral, eliminating hydrostatic resistance braking force, you're toast. Even if in gear, the same uneven traction would prevent the hydrostatic drive resistance from being able to apply a braking force to the wheel with traction. The same thing would happen.
All of the above described is not a noticable factor on flat level ground with generally even traction to both wheels. You would never know the diff lock has failed on generally flat level ground because the wheels still get generally even braking. Compounding this is how infrequently the brake is even used at all due to the hydrostatic drive. It would be VERY VERY easy to not know your diff link linkage is broken until you're in a situation where you actually need it... like uneven terrain going down a grassy hill with the transmission accidentally in neutral.
One front tire and one rear tire always have power or braking, It increases to 2 rear tires plus one front when the differential lock is engaged.