Look at the top of the phone. Next to the headphone jack, do you see a thin black line (the antenna) going across the top? If so, you have an iPhone 4, not 4S.
There are some subtle differences between certain generations of iOS devices which you can use to tell them apart. For instance, iPhone 4 and 4S look very similar, as do the iPad 2 and The New iPad.
iPhone 4 iPhone 4S
The antenna (black line) is located on top of the iPhone 4, but does not appear on the iPhone 4S.