I did not say ALL 4x4 are optional, I said usually. Let’s break my argument down to basics. A traditional and full 4x4 application offers the user a selection of Low, High and Auto modes. An AWD system is just that; the system can apply power to 1, 2, 3 or 4 wheels. The auto mode in 4x4 applications splits the power fore and aft. It mimics AWD characteristics by applying brakes to wheels experiencing less than optimal traction. The differential setup in 4x4 is usually very different from AWD systems.
The differences are subtle and are not just limited to semantics. In the US, Ford and GM do not mark any of their vehicles 4x4 other than trucks. Chrysler has to protect their Jeep brand from complete annihilation from very good products from other US and foreign competitors. They mark those vehicles with 4x insignia as a marketing ploy. They make other SUVs with the same power train as Jeeps that are not marked at all.
Here is one link: http://www.4x4abc.com/4WD101/difference_4WD_awd.html
I am sure you can use any search engine to find 100s of other explanations.