My wife's work phone is a new IPhone 4Gs. In the three months we've had it I'm yet to find a 4G signal. we live about 20 miles outside of Boston and have travelled to six states and have received no better than a 3G signal. The phone is amazingly fast but the signal indicator receives only a 3g signal here. Are you sure your getting a 4G signal outside the immediate area of a large city. I doubt it. Right now 4G is a scam as there is no signal outside of large cities.
The link below is the ATT 4G map coverage are the orange circles, below that is the Verizon map signal is in the yellow circles. What the phone companies are doing is selling a product they don't provide to raise enough $$$$ to provide it in the future.
att map just the orange spots