The Apple event yesterday, 10th September, saw the discontinuing of the iPhone 5. The iPhone 4S is still there along with the new iPhone 5S and the iPhone 5C. To help you with your buying decision, Apple has created a comparison chart of the three models that you can view on their website.
One of the key factors when choosing which iPhone to buy is going to be the price. The iPhone 5S starts at $199 subsidized by a two year contract and this runs all the way up to $399 for those who are looking for the high end 64GB model. The iPhone 5C is priced between $99 and $199 which is considerably different and the iPhone 4S costs nothing on contract. With pricing out of the way, it will boil down to internals and specifications.
If you are looking for LTE you will have to choose between the iPhone 5S or the iPhone 5C as the iPhone 4S doesn't support this. LTE is going to future proof your device and purchasing a 4G LTE compliant model is definitely advised.
The iPhone 4S comes with a very capable A5 chip whilst the iPhone 5C comes with the A6 chip. The A7 chip is a big step up and this is available in the iPhone 5S. Animations will be smoother and every day tasks will be much quicker. If cameras are your thing, you should take note that the cameras have gotten progressively stronger since the iPhone 4S and iPhone 5C has the same camera as the iPhone 5. The iPhone 5S features True Tone flash which brings more realistic lighting in dark situations and is generally better than the iPhone 5.
It is strange to see the iPhone 5 being removed from existence and is now basically obsolete. What's left for you to decide now is what iPhone you choose to buy. Again the full specs comparison table is available on the Apple website and worth taking a look at before you make your decision.