All four factors are important, but if one factor was the most important, it would be the character and purpose of your work. Modern courts typically look at how "transformative" your work is when analyzing this factor. Certain types of uses, like selections of a movie for education or review, are generally considered to be suitably transformative. Even keeping something in the same format (for instance a song parodying another song) might still be transformative. The key question here is "Have I added new and original meaning or understanding to the original work?"
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