Buy the unlocked 4S a few weeks from now in November. (Yes, Apple will be selling it officially!)
Use either the AT&T or T-Mobile prepaid plans. (You are pretty much guaranteed to save money over the cost of a two-year contract.)
Sell your 4S on Ebay after a year, and buy your new phone.
Voila. First-world problem solved. (This is what I'll be doing, anyways.)
Bonus: if AT&T or T-Mobile turn out to have bad reception in your apartment, you can effortlessly switch to the other carrier.