## 2 Materials and Methods
### 2.4. Energy between Lean and Industry 4.0
(2017) introduced the concept of \"Cyber Physical Just in Time\" [83] by associating Industry 4.0-specific CPS technology with the basic JIT concept of Lean management. [PERSON] (2019) [84] considers that Industry 4.0 facilitates a Lean and green supply chain with environmentally friendly implications. Research in the field has often provided models that are difficult to adapt to practical applications in a real industrial environment [85]. Given this research gap, this paper aimed to contribute by applying Lean and Industry 4.0 to flexible manufacturing. The difficulty resides in the fact that there is currently no unanimously accepted integration model in industrial organizations. The model developed in Reference [62] proposes the grouping of 14.0 components according to the characteristics of various systems, processes and technologies. Many researchers and managers ask themselves questions regarding the compatibility between the already applied Lean principles and the \"new paradigm\", Industry 4.0, and the possibility of correlating the two. Challenges are also noted regarding the ability of Industry 4.0 to implement Lean principles and techniques and, vice versa, the adaptability of these principles to the new paradigm of Industry 4.0 manufacturing [86]. Analyzing and evaluating good manufacturing practices in several manufacturing companies (SMEs) in Germany, [PERSON] et al. (2017) believe that Industry 4.0 can provide \"high end solutions which possess the necessary tools to implement Lean\" [86]. However, at the same time, they specify the fact that the majority of research is theory oriented and not applicable, and they recommend the elaboration of a conceptual framework to integrate cybernetic systems with Lean manufacturing. An interesting point on this subject is discussed in [24], which emphasizes that \"Lean production and Industry 4.0 are not mutually exclusive\" but, on the contrary, could be integrated, even if there currently is no unanimously accepted idea in this regard. [PERSON], [PERSON] et al. (2020) [46] examined the application of the Kaizen principles of continuous improvement and Lean Six Sigma and concluded, by combining the solutions offered by Industry 4.0 with the methods of continuous improvement, that it can bring important benefits and present a conceptual model of integration. According to certain researchers [17; 62; 85], the Lean system represents a foundation for implementing Industry 4.0. One of the first integration models [64], proposed by Sony (2018), considered that Industry 4.0 technologies, structured along the three directions of \"integrated vertical, horizontal and end-to-end engineering integration\", could be superimposed upon Lean principles. [PERSON] et al. (2018) [87] proposed \"Lean automation\" by introducing autonomous AGV and Internet of Things (IoT) I4.0 technologies, while highlighting the managerial and social implications. Investigations into the automation of Lean production by applying I4.0 technologies were initiated by [PERSON] et al. (2015). Industry 4.0 and Lean were considered to support and complement each other [88]. A literature review regarding the development of the two manufacturing concepts aimed to analyze the current principles and relationship between Lean management and Industry 4.0 for the purpose of generating a useful conceptual integration framework applicable to industry. In the analyzed context, there is a need and opportunity to develop in-depth research in this cutting-edge, booming field in order to identify the benefits regarding the increase in system performance. Mainly, this concerns the synergistic development of digitalization and connectivity in Industry 4.0 and the Lean principles to ensure efficient, sustainable and waste-free manufacturing in the future.