4. To which the fourth book of Kautilyas Arthashastra deals with?

The suppression of criminals and other anti social elements

5. What are the similarities between Kautilyas Arthasashtra and Platos Republic?

The Arthashastra does not look upon politics as a branch of
philosophy. It keeps personal morality distinct from
public good and it is thoroughly practical and is only
concerned with the problems of governance of a state