Integrating sustainability, a flexible manufacturing system, and Industry 4.0 resolves the issues of fluctuating market demand arising from customization requirements. Modular products allow flexibility to adapt to changing requirements and optimize resource utilization. In this study, a method was proposed and applied to two products, i.e., a 3D printer and an electric toothbrush featuring modular architecture, multiple product versions, and customization, to contribute to the development of sustainable flexible manufacturing systems. From the results of the two case studies nine modules were identified that contain specific functions and related interface information. From these modules, one platform was developed that comprises common entities used in all variants of the products. This platform was further extended to product families. From the modules, product architecture was developed that supports the product and process relationships. These relationships can be developed concurrently, enabling product features to be linked to the manufacturing setup. Thus, when a modular architecture is developed, the factory has to be reorganized accordingly, or reconfiguration is possible. Hence, the main aim of the research was to develop modular product architecture to identify product and process relationships for a sustainable flexible manufacturing system.