Not too long ago, Apple pushed the iOS 5.1 software update for iDevice users through both over the air (OTA) and iTunes. The update brought the 4G indicator to the AT&T iPhone 4S as it uses high-speed HSPA+, which technically is true 4G according to the International Telecommunications Union (ITU). Other iPhone 4S owners on different cellular networks along with other iPhone model owners still get the 3G indicator as AT&T is the only company in the US licensed to advertise 4G with the iPhone at this point in time.
If you’re one of the Verizon, Sprint, or unlocked iPhone 4S owners, or you own the iPhone 4, then you have the option to display the 4G indicator regardless of the fact that you don’t truly have 4G access. This is done with a free WinterBoard modification in Cydia dubbed 4G for iPhone 4 by theme creator lordjordysmith. You can find it in the ModMyi repository:
We emphasize the fact that this modification is only cosmetic and to mess with your friends into thinking you have 4G access. This modification does not increase your data speeds to 4G speeds and it does not give you any kind of advantage over other 3G-using iPhones. The 4G indicator will show whenever you’re connected to a 3G network, but when you connect to a 2G (Edge) network, you will still see the E indicator as usual. If you are connected to a Wi-Fi network, it will show the Wi-Fi icon as usual.
To enable the modification after installing it, search your home screen for the WinterBoard application and tap on the 4G for iPhone 4 theme. Once you do, a Respring button will become visible in the top left-hand corner of the application – tap on it to save your changes and watch as your iPhone 4 starts displaying the 4G indicator.
This modification does not work on the iPad or iPhone 3GS as it is optimized to work with the retina display of the iPhone 4 and iPhone 4S only. The iPod touch does not have cellular data, therefore it doesn't work with that device either.