Ad 4 - country can simply compare the prices of goods they export. If you export iron for 10 times the price of grain, and other country does so for 8 times the

Ad 4 - country can simply compare the prices of goods they export. If you export iron for 10 times the price of grain, and other country does so for 8 times the grain price - you have comparative advantage in grain and the other country - in iron.