If your carrier does not provide free hotspotting, then you'll need to jailbreak your device and install tweaks such as TetherMe to ghost tether your device. The iPhone 4S can only be jailbroken on iOS 5.0.1, 5.1.1, 6.0, 6.0.1, 6.1, 6.1.1, and 6.1.2. None of the other listed iOS can be jailbroken for the iPhone 4S. You cannot downgrade your iOS.