Question

How can I support game center for iOS 4 but still run app on iOS 3. So game center will available only on devices with iOS 4.

Answer 1

You should make a weak link between gamekit framework and your app. You can make weak link with any framework in your project’s settings. Also check apple game kit programming guide for checking if game center is available. Basically it boils down to single function

BOOL isGameCenterAvailable() {
    // Check for presence of GKLocalPlayer API.
    Class gcClass = (NSClassFromString(@"GKLocalPlayer"));

    // The device must be running running iOS 4.1 or later.
    NSString *reqSysVer = @"4.1";
    NSString *currSysVer = [[UIDevice currentDevice] systemVersion];
    BOOL osVersionSupported = ([currSysVer compare:reqSysVer options:NSNumericSearch] != NSOrderedAscending);

    return (gcClass && osVersionSupported);
}

as well as including GameKit/GameKit.h header file in your prefix.pch file.