Generally speaking, using 4x4 only as needed is best.
On high-traction surfaces, like pavement, or rock, or concrete...the part-time 4x4 system of these tractors will bind upon turning and it puts a large amount of stress on the drivetrain. This is because as the front wheels turn, they are traveling at a different speed than the rear wheels (the inner wheels have a shorter distance to travel than the outer wheels). While there is a front and rear differential on each axle, there is no differential *between* the axles, and so no way to adjust for different speeds of the front and rear axles. When you turn in 4x4 on a highly tractive surface like pavement, your tires are forced to slip after the drivetrain binds. This is no different than the part-time 4x4 system found in many modern pickups. When you are on slippery surfaces such as grass, dirt, snow, etc...the slipping still happens...but it happens very easily with virtually no binding or stress on the drivetrain.
When your tires slip (and it usually is always the front tires to slip since they are smaller and gain less traction)...they wear. So although if you leave your machine in 4x4 at all times more than likely nothing will actually break...the tires will wear much faster.
I wish Kubota would offer a 4x4 indicator on the dashboard, but they don't on most models.