What is the brand of your non-apple tv?!!
Do you have to have an Apple tv in order to connect the iPhone 4S to it?
Yes indeed you need an AppleTV in order to connect anything to it. If you question was how to plug your iPhone to a TV without the AppleTV, then yes, you will need the proper cable like these; http://store.apple.com/us/product/MD098ZM/A/apple-digital-av-adapter or http://store.apple.com/us/product/MC748ZM/A/apple-composite-av-cable?fnode=45&fs =m.iphoneCompatibility%3Diphone5 depending of your TV.
Note the price itself of such cable as you may think again if an AppleTV would worth the purchase as you'll not need any cable.