They would have to cut off trade to much of the west as well, otherwise exports would simply be redirected. They could do USA did with Iran, and refuse to trade with anyone hat trades with the USA, but that is a losing proposition for them.

Basically, china cuti off trade with the USA would be a North Korea style action.

Does that include all the value created locally in order to be able to make those exports? Like food that people eat, vehicles that they have to drive, etc., That much drop in economic activity will surely trigger much worse outlook than you are alluding to.

No, not directly, but indirectly people would be afraid of further radical policy changes by China. Investors would move their money out of China. Or at least, that's what I'd do.

4% by population of course, which is what you meant, but I think 40% of the world by wealth. While I understand what you mean by saying there are new middle classes being created from the other 96%, it seems like, absent another even more catastrophic economic screwup than we put ourselves through 8 years ago, American domination of the world market isn't going anywhere any time soon.