As of the Foursquare 5.0 release, we are no longer supporting devices with iOS3 nor iOS4. If you can upgrade to iOS5, you’ll be able to get new bug fixes and updates.

You’ll still be able to run the existing version of Foursquare on your phone (do not delete it!), but you will no longer be able to grab updates. (If you accidentally delete Foursquare from your phone and you’ve previously backed up your phone using iTunes, you should be able to restore the app.)