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If your phone is receiving a 4G signal, '4G' should appear in the status bar.....though this may vary with country, and possibly device. The number of bars does not indicate the type of signal, such as four bars for 4G. The number of bars gives a rough indication of signal strength, with a maximum of usually five. For greater accuracy, you could go to Settings, About Phone, Status, Signal Strength.
If your phone is receiving 3G, it may say '3G' or 'H' or 'H+' in the status bar, depending on the type of 3G signal. At least, this is how it is for me in the UK....there may be variation. Again, the number of bars would indicate the strength of that signal. A strong 3G signal may be more useful than a weak 4G one.
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