Steve unless you need a better camera or Siri what a waste of money mate keep the 4 and wait for the next iphone! seriously dont get anyone that would go from a iphone 4 to a 4s!!
A5 chip though dual core 
9 months time, A6 quad core. Keep the 4.
Why not wait till 2013 for the A7 chip, or sod it actually; 2014 for the A8!
Technology moves quickly, waiting for the next thing that will be out of date in the same time frame is pointless. If you want something get it. Even my missus wants a 4S now as its so much faster, the internet is silly quick in comparison to her 4!
I dont think the 4S is worth getting for 4 owners, though Siri is superb, its better for 3GS owners. The 5 wont be worth it for 4S owners and the 5S wont be worth it for 5 owners but will be for 4S owners etc ad infinitum
If you can afford to do it or have an upgrade then go for it, as a package its the best phone on the market at the moment.