Given your personal background, describe an experience that illustrates what you would bring to the diversity in a college community, or an encounter that demonstrated the importance of diversity to you.
Your Harvard application will reveal to the Harvard admission committee what you can contribute to the institution. You need to give them a reason to choose you over the many other applicants. If your Harvard application is indistinguishable from the applications of others, you will have a hard time making it past the Harvard admission process. Make whatever efforts you can to seem unique. Get personal statement help if you can, for instance.
Question 4b explicitly refers to the fact that the university is looking for good diversity in its student population. It is asking how you personally would contribute to this diversity. Here is a sample answer for your reference.