IS your city on the list of 4G area's or are you on the fringe?
I was running some speed tests on the way home last night. I work in a 4G area and eventually hit a 3G area.
When I was in the city I was getting about 10-12Mb/s download speeds. As soon as I got out of a heavier populated area my speeds jumped up to 18-20Mb/s.
What I found to be the biggest issue were users and of course terrain. When I would be dipping into a small "valley" (for lack of better words) my speeds would rop into the 4-9Mb/s range but my phone would not switch into 3G mode.
So I am guessing you are not fully enveloped in a 4G area.