If the 5S comes with a 4.3 inch screen and a decent increase in battery life (not to mention the rumored increase in capacity at same price), it will make my decision to upgrade a no brained. I currently have an 8 gig iPhone 4 and I need more speed, larger screen and have very little space left.
With only a 4 inch screen that doesn't even offer a wider screen, I'll be thinking seriously of skipping an upgrade and waiting for the iPhone 6.
I honestly think in your position no matter what the iPhone 5S offers it won't be nearly as good as a jump from your 4 -> future 6. I'm holding out for the 6, and I would really welcome a larger screen as well!