StackExchange Rank in Resume? 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?
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.