Addressing the lack of systematic review in existing literature on the theoretical aspects of four-network integration, particularly its connotation of four-network integration, this study first proposes that the connotation of four-network integration mainly includes multi-level rail transit classification and multi-level rail transit integration, which complement each other. The deep requirement of multi-level rail transit classification is to ensure efficient coordination of the "four networks," with a hierarchical structure that facilitates the balanced development of passenger flow. The focus of multi-level rail transit integration development can be subdivided according to the implementation path into planning and construction integration, operational service integration, and management mechanism integration, ultimately achieving temporal, spatial, and functional integration. Secondly, based on the connotation, this paper analyzes two domestic cases: the cross-platform interchange at Chengdu's Xipu Station and the cross-line operation between Chongqing Rail Transit Jiangtiao Line and Metro Line 5. Thirdly, it examines the challenges currently faced in promoting four-network integration in China, specifically including unreasonable network structure allocation, lagging planning and construction coordination, insufficient coordinated service levels, and deficient coordinated institutional mechanisms. Finally, targeted strategic recommendations are proposed in four aspects: rail transit service demarcation for the "four networks," planning and construction integration, operational service integration, and management mechanism integration, aiming to provide theoretical support and practical guidance for the sustainable and healthy development of four-network integration in China's rail transit.