Capital deepening refers to the process of increasing the capital-output ratio, which means that there is a greater amount of capital being used for the production of each unit of output. This can be achieved by either increasing the amount of capital per worker (option 1) or by improving the efficiency of capital usage (option 3).

Option 2, emphasis on social overhead capital, is not directly related to capital deepening. Social overhead capital refers to infrastructure and public facilities such as roads, bridges, and schools, which support economic activities but do not directly contribute to the production of goods and services.

Option 4, increasing the capital-output ratio, is the correct answer. This implies that more capital is being used to produce each unit of output, which can lead to increased productivity and economic growth. Capital deepening is often seen as an important driver of long-term economic development and can result in higher wages, improved living standards, and technological progress.

Therefore, the correct answer is option 4, increasing the capital-output ratio.