Well, as long as you are comfortable with it.
When I said "look at the four facts," maybe I should have been more clear -- look at the four factors in light of current caselaw. The caselaw makes it clear that the four factors are not always weighted equally, that "effect on the market" is probably the most important factor, and has a lot of sub-factors, including whether or not the copyright holder might want to enter the market that the potentially infringing work is in. You might want to start with the Wikipedia page on fair use, then read up on the cases cited in the article, to help better inform your decision.