Part 4 is more thematically unified than Parts 2 or 3 and explores commercial law and its history. It is divided into four books: on the nature of commerce, on its revolutions in world history, and the use of money, before taking a left turn to discuss population size and law.

Book 20, “on the laws in their relation to commerce, considered in its nature and its distinctions,” lays the groundwork for Part 4. Therein, Montesquieu outlines the spirit of commerce and its effects on the mores of commercial peoples. He writes approvingly of commerce that it “cures destructive prejudices, and it is an almost general rule that everywhere there are gentle mores, there is commerce and that everywhere there is commerce, there are gentle mores” (338). The value of commerce in pacifying societal mores is unparalleled in its universality. Commerce engenders a mutual need between two countries reciprocally engaged in commercial enterprise. Commerce also requires and reinforces peace amongst nations and is, in Montesquieu’s view, antithetical to war. Different forms of ideal government are differentially committed to economic activity, with republics typically being more commercial. Montesquieu points to England throughout Part 4 as exemplary of commercial enterprise.