Why won't Hands-Free Control work for iPhone 4s?

I just read an iPhoneHacks article that seems to indicate this app will work on iPhone 4s. When I try to buy it, I am told it will not work on my IOS version (8.3). I am wanting this app so I can answer calls and hang up with a wave of the hand. What do I need to make this app work, or is there another app out there that will do the same thing?