The connection with earlier topics seems to be this, that the natural progress of wealth is from agriculture to manufacture - food is the first thing that has to be set aside before anyone can specialize in manufacture, and town manufactures use materials from the country. Book 3 explains why in Europe this was not so. Division of labour made more progress in town industries at first, because for political reasons the kings fostered self-government, and thus better government, in towns, while tyrannical landlords inhibited industry in the countryside. Thus book 3 prepares the way for book 4, by showing the importance of political factors in the development of the wealth of nations. The management of a household involves especially getting supplies.
Political economy is the corresponding art in relation to state.
The two main theories of political economy with which book 4 is concerned are usually called the 'mercantile' and 'physiocrat' theories. The first was the theory that a nation's wealth is increased mostly by export industry, the other that it is increased mostly by agriculture. Both theories are still around. In fact this conclusion does not establish itself of its own accord, as soon as systems of preference and restriction are refuted. Book 4 refutes theories proposing that industry should be directed especially into export or into agriculture.