This is a new badge, especially for today, April 16th which is Foursquare Day (16 is the square of 4). Foursquare have been very secretive about how you unlock it but I earned mine by checking in at any location and shouting 'happy foursquare day' in the text box. As long as you do this today, you should earn the badge. If anybody has any different ways of earning it, let me know. When you do earn it, you get the message: "Happy foursquare day! (four squared = 4/16, get it?) Here's a toast to all of you for making foursquare so amazing and to the 50+ 4sq Day parties going on around the world!"