Why doesn't my iphone 5 have 4g?

I brought my iphone about 3 weeks ago and I got 3G working because I thought that was what it just had, but I was watching the tv and it said that the iphone5 has 4g, so why does mine only say 3G in the top status bar

Depends on your plan and your service priovider, contact them and ask them.

It could be disabled. To enable go to:
Settings -> General -> Cellular -> Enable LTE

However, you may be in an area that does not have 4G LTE coverage. Did you get a Sprint iPhone? If you did I have bad news. Sprint is awful. If you got a Verizon one you should LTE in most places and are just unlucky to live in a 3G spot.AT&T iPhone 5s use HSPA+ and called their 3G "4G" so I know you don't have that model.