Explanation:

- The Central Board of Directors of the RBI is appointed/nominated for a period of four years.

- Option 1: five – This is incorrect; the tenure is not five years.

- Option 2: six – This is also incorrect; directors are not nominated for six years.

- Option 3: three – Not accurate for Central Board of Directors; three years applies to some other positions in government bodies, but not here.

- Option 4: four – This is the correct answer. As per the Reserve Bank of India Act, the directors of the central board are appointed for a period of four years.