iOS 5 may have been difficult to download, but it's worth the wait — especially for Foursquare users, who got an update today that could change how they interact with friends and places. In version 4.0 of its iOS app, Foursquare has added a real-time feature called Radar. Radar uses your location to tell you when you are near your checked-in buddies, something you've marked on your to-do list, or an item on a list that you've subscribed to. By notifying you of convenient opportunities to hang out, the hope is that next time you're out and about you'll choose meet up with a nearby friend. Since Radar allows Foursquare to move away from its reliance on manual user check-ins, the company sees this as a major step in the advancement of its service. Like Reminders, Apple's location-sensitive to-do app, Radar works by simply pushing notifications to your iPhone. Update 4 is already available from the App Store, though only iPhone 4S and 4 users running iOS 5 get Radar. No word yet on Android availability; they'll just have to make do with Google Latitude for now.