Like Tim K said, the most reasonable explanation would probably the area you are currently in does not have 4G. I have seen many people who do not have 4G and it is simply because the area they are in has low or no 4G coverage. When you are not in a 4G area, your phone will resort to EvDO(EV) which is basically 3G. If you are not sure if you are in a 4G area, you could always check your carrier's website. Another reason could be that your carrier is working on 4G coverage or other service in your area.
Hope I helped.