1. What is the character of the use? Is it "transformative" so that a new work or new use is created?
2. What is the nature of the work to be used ? Facts and published works are "fair use" more than imaginative, unpublished ones.
3. How much of the work will be used ? Small amount or an appropriate amount for transformative purpose is better.
4. Effect of Use Upon the Potential Market. Would your use deprive the copyright owner of income or undermine a potential market?