第四阶段是工业化后期，城市间的边缘地区发展很快，区域性基础设施以及工业

The fourth stage is the late stage of industrialization. The fringe areas between cities have developed rapidly. Regional infrastructure and industrial satellite cities have developed rapidly. The economic, cultural and technological connections between cities are relatively deep and wide, with high density and heavy load. The feedback function has formed an integrated network, namely the urban agglomeration. <br>Friedman’s urban agglomeration theory uses the development perspective to analyze the formation of urban agglomerations, demonstrates the dynamic process of urban agglomerations, reveals the hidden power behind urban agglomerations, namely the high concentration of regional productivity, and emphasizes the impact of economic growth on urban agglomerations. The important role has opened up the economic analysis of urban agglomerations. At the same time, it has also begun to pay attention to the role of industry on urban agglomerations, and has done basic work for subsequent research.

The fourth stage is the late stage of industrialization, the marginal areas between cities develop rapidly, regional infrastructure and industrial satellite cities develop rapidly, the economic, cultural and technological links between cities are relatively deep and extensive, high density, heavy load, resulting in the interaction and feedback of cities, forming an integrated network, that is, urban groups.<br> Friedman's theory of urban clusters uses the perspective of development to analyze the formation of urban clusters, shows the dynamic process of urban clusters, reveals the high degree of cohesion of urban clusters hidden behind the power of regional productivity, emphasizes the important role of economic growth in urban clusters, opens up the economic analysis of urban clusters, and also begins to pay attention to the role of industry in urban clusters, and does basic work for follow-up research.

The fourth stage is the late stage of industrialization. The marginal areas between cities develop rapidly, regional infrastructure and industrial satellite cities develop rapidly. The economic, cultural and technological links between cities are deep and wide, with large density and heavy load. The mutual attraction and feedback of cities form an integrated network, namely urban agglomeration.<br>Friedman's urban agglomeration theory analyzes the formation of urban agglomerations from the perspective of development, reveals the dynamic process of urban agglomerations, reveals the hidden driving force behind urban agglomerations, that is, the high concentration of regional productivity. It emphasizes the important role of economic growth on urban agglomerations, and opens up the economic analysis of urban agglomerations. At the same time, he also begins to pay attention to the role of industry on urban agglomerations Basic work has been done.<br>