There is a 4G iphone? since when?
iPhone 4S is effectively 4G on AT&T. Note AT&T 4G is not as fast as Verizon 4G. You can research this yourself. AT&T has somewhere in the 12MB download range (max situation) where Verizon has approximately a 20mb download speed (max).
The reason the 4S exists as it does today is because the 4G system Verizon employs required a separate chip to do 4G and there is no room in the phone to put a second chip. The 4G uses a single chip for AT&T 3G and 4G speed.
The iPhone 5 will have a new hybrid chip for Verizon that works both 3G and 4G.