What version of iOS can iPhone 4S run without having to jailbreak?
Apple's website for iOS explains that the (now, as of February 2013) current version of iOS runs on all but the first two models of the iPhone. So the 3GS, 4, 4S, and 5 all support iOS6. As to what will happen in the future, no one can say besides Apple, but they currently support the latest four generations of iPhone hardware (but only the latest two for the iPod Touch).
Apple's website previewing iOS7 indicates that iOS 7 will support iPhone 4, 4S, and 5, the 5th generation iPod touch, the iPad 2, the iPads with retina display (two models, one with traditional dock connecter and one with Lightning connector but only listed once on their site), and the iPad mini.