Quote 4
In coming to a fixed determination to run away, we did more than Patrick Henry, when he resolved upon liberty or death. With us it was a doubtful liberty at most, and almost certain death if we failed. For my part, I should prefer death to hopeless bondage. (10.26)
Remember Patrick Henry? He was famous for saying "Give me liberty or give me death" during the Revolutionary War. In the preface, Garrison already compared Douglass to Patrick Henry, but when Douglass brings up the reference he's saying that he and his slave friends are even braver than one of the fathers of the country.
Quote 5
Such will try to discredit the shocking tales of slaveholding cruelty which are recorded in this truthful Narrative; but they will labor in vain. Mr. Douglass has frankly disclosed the place of his birth, the names of those who claimed ownership in his body and soul, and the names also of those who committed the crimes which he has alleged against them. His statements, therefore, may easily be disproved, if they are untrue. (Preface. 10)
William Lloyd Garrison knows that Douglass's enemies will try to claim that Douglass's narrative wasn't true. (And they did.) Pro-slavery debaters would try to deny or downplay the horrors of slavery, so Garrison understood that all Douglass really had to do was tell the truth. By encouraging Douglass's opponents to try to disprove him, he showed that he and Douglass had nothing to fear from the truth.