You can see this link for more information on Objective-C libraries that work with the fourth-grader APIs.
Edit: Currently this one seems fresher, more likely to work correctly, and also good documentation on how to integrate it. This is the second one that is listed on the four-room library page.
Also check the foursquare developer site as a whole, you need to know the different endpoints and what data they return anyway => https://developer.foursquare.com/
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