Unless it cost me $100 or less, I would not switch either. Although the 4S is much quicker than the 4 at games, navigating through menus, etc. other than the improved camera and Siri as Unleashed stated, it's not worth extending a contract or spending much on. Just my 2 cents.
And no one knows when the 5 will arrive. They are only making guesses based on rumours. If history repeats itself, I would guessitmate it will happen before the end of the year. I don't think it will happen in the Summer as 4S sales are too strong. Why re-tool when you are making paper......