How we cite our quotes: (Book.Chapter.Paragraph.)
Quote #4
In other parts of Gramarye, of course, there did exist wicked and despotic masters—feudal gangsters whom it was to be King Arthur's destiny to chasten—but the evil was in the bad people who abused it, not in the feudal system. (S.14.3)
These other masters are juxtaposed with Sir Ector, the benevolent feudal master. So, it's not the feudal system itself that's bad—it's the way the individual masters use their power for good or for tyranny. Arthur makes it his life's goal to figure out how to channel the use of Might for good.
Quote #5
"Only fools want to be great." (S.20.19)
With greatness comes great responsibility, Merlyn is implying to Wart. Those in positions of power either run themselves ragged by trying to do the right thing all the time (which Arthur ends up doing), or they turn tyrannous and end up like the many bad and bloody barons the King has to whip into shape.
Quote #6
It was the unfairness of the rape of their Cornish grandmother which was hurting Gareth—the picture of weak and innocent people victimized by a resistless tyranny—the old tyranny of the Gall—which was felt like a personal wrong by every crofter of the Islands. (Q.1.57)
Here, White links the sexual power brutally and unjustly used against Igraine to the political power brutally and unjustly used against the Gaelic people by the Gallic people (the ancient ethnic feud that the text returns to many times). It's kind of a conventional trope to describe the conquest of a nation or people in terms of the rape of a woman.