Need more info. What size is the iPhone 4 (Thinking about resale value). Are you still under contract? Do you have to pay an ETF?
A quick search of ebay shows the used 16GB 4 on at&t selling for about $250. Another search shows a 16GB Verizon 4S is around $375-$400.
So what if you were to sell your 4 and combine the proceeds with the money that you would have invested in a new 4S and buy a used one. So conservatively, sell your 4 for $200, take the $200 you would have spent on a a 4S with activation and you have $400 to buy a used 4S.
Then you will still have an upgrade available for whenever the next iPhone is released. You will be able to sell the 4S and at least cover the cost of the upgrade. When all is said and done, you have spent the same $200 out of your pocket now, you have a 4S to use in the meantime, and will have the next iPhone when it comes out.
Just a thought.