John Locke, Voltaire, John Stuart Mill and Bertrand Russell are herein credited with advancing the cause of tolerance. Popper makes the case for tolerance following Voltaire who argued from fallibility that we should stand ready to forgive others around us - and therefore be tolerant for humans make errors. We discuss what "interpretation" meant to Bacon (it is quite the opposite to what it means today to most people most of the time) as he speaks of interpreting nature. So does this make him an early Popperian? Socrates "maieutic" (the "Socratic method" - his means of elucidating knowledge by the asking of careful questions) seems to come in two versions: that designed to uncover absolute truth and that with more of an emphasis on correcting errors. In this fourth part we are really getting a deep lesson in philosophy from Karl Popper himself through his summaries and analysis of the greats in the Greek and British/European philosophical traditions.