Is the drivetrain with the 4A the same as the drivetrain with no 4A. If they are the same, do the system simply switch automatically between 2H and 4H ?

 

The 4A system uses a different transfer case with a clutch that allows the front and rear driveshafts to spin at different speeds as needed, this is why you can safely use 4A on dry roads. The non-4A transfer case with 2H, 4H, and 4L will lock the front and rear driveshafts together in 4H and 4L, so you can only use 2H on dry roads.