Module 4 introduces Input-Output (I/O) analysis as a framework for modeling interdependencies between economic sectors. It explains the structure of I/O tables and demonstrates how to solve those models by deriving the Leontief Inverse. The module notes the model's reliance on the Leontief production function, which assumes fixed input proportions. Finally, it covers applications such as modeling economic shock propagation, distinguishing between forecasting and dynamic modeling, and extending the model to analyze environmental impacts and technological change.