Quote 4
"I want you to show them the difference between what they think you are and what you can be." (24.40)
Grant isn't asking Jefferson to just be better than people expect him to be. He's asking him to teach them a lesson about prejudice by showing them how wrong their expectations are about him. Only if they are made aware of the difference between their expectations and reality will they start to question their prejudices.
Quote 5
"Their forefathers said that we're only three-fifths human—and they believe it to this day." (24.43)
Grant is referring to the Three-Fifths Compromise, a decision between northern and southern states to count slaves as three-fifths of a human being for purposes of taxes and number of representatives in the House. That political decision helped cement prejudices about the worth of black people in the minds of Americans.
Quote 6
"Do I know how a man who is supposed to die? [. . .] Am I supposed to tell someone how to die who has never lived?" (4.105)
This question shows that, even if Grant doesn't know what it is, there is a proper way to meet your death and face your mortality. The problem is that death is the end of a life, and he doesn't consider that Jefferson has really had much of a life at all.