But there is a workaround for that, yes your non AT&T iPhone can get this 4G indicator to replace the 3G indicator and again Cydia is a rescuer to deal with the situation. You can install 4G for iPhone package that it's currently stored in ModMyi repository, but firstly this package requires you to install Saurik’s theming platform. Once you've installed this free of charge tweak you will be seeing 4G indicator like in the following picture.
Keep in mind the indicator it is simply a visual indicator and replacement of 3G indicator, does not offer to increase the speed of your connectivity. Interested? Then launch Cydia to grab the 4G for iPhone 4 or iPhone 4s.
Hope this post useful !