I'm ranked as one of top 4% users of Data Science Stackexchange. I'm applying for a PhD program in related field.
Shall I mention my Stackexchange rank in my resume? If yes under what title?
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Yes, it is worth including in your PhD application somewhere, if you are applying to a department where data science knowledge would be a significant asset. Don't expect it to receive a lot of weight, but it can't hurt.
Many PhD applications allow you to include a link to your web page(s). I suggest you include a link to your Data Science profile page as one of these links, so members of the admissions committee can see example questions and answers you've written there, if they wish.
I don't know if I'd include rank as a percentage, as it's hard to evaluate what 4% means or how many users there are there. In your case, I would seriously consider listing it as 13th highest-rated user (out of N users) on Data Science Stack Exchange, a question-and-answer site for technical questions related to data science.
I also agree with Wolfgang that rather than merely mentioning your involvement, it might also be helpful to have an explanation or narrative about why you believe this makes you a stronger candidate. To give some examples: Does it provide you with valuable knowledge about how to do research in your area? If so, can you give a specific example? Did your involvement lead you to become enthusiastic about research in your chosen area? Perhaps you are especially motivated by bringing ideas from the research literature to a broader audience and helping practitioners benefit from the research literature, and your involvement in Data Science.SE is motivated by that? Members of the admission committee might not be familiar with this site or why your involvement is relevant to the admission decision. Give them a concrete reason why this makes you a better candidate for admission or what this says about your interests and preparation.
You may mention it, but it's not going to carry a lot of weight. What I'd focus on would not be the particular rank you have there, but that you have been active in the community of Data Scientists for a good while and believe that this has given you a good overview of this area. That's what matters, not what you do in your spare time.