A company by the name of Pixeon is a Swiss manufacturer thathas developed a new Smartguard case for the iPhone 4/4S that combines a can of pepper spray right attached to a iPhone case.
The Smartguard case is molded in hard plastic and has a spot on the side for the canister of pepper spray to lock-in. This will allow you to quickly access the pepper spray in an emergency situation. It will also ensure you don’t accidentally discharge the canister in your pocket.
The Smartguard case is available for $34.99 and can be bought here