I ordered an iPhone 5c for 4G usage. However, the phone I received displays a 3G on the top left of my screen.
Does 3G on the top left mean I'm using 3G?
Does this convert to 4G through proper configuration?
If you see a 3G at the top left it means that you are in an area which does not have 4G support.
When you are in an area with 4G (or, on Verizon, LTE) you will see that in the top-left corner instead.
iPhone 5c or 5s definitely support 4G or LTE and it clearly displays the 4G symbol. If it does not, it means your mobile service provider is not providing the 4G service or even if it does, maybe in some geographical areas the 4G service is not available.