I think it has something to do with the iphone being unlocked. I've heard that unlocked phones kind of suffer a little more because its not locked (in the closed loop of the network if you will)
I had an unlocked non-smartphone and the reception was not as good as the locked phones. You might not be able to get it.
You can unlock your phone if you like but your Iphone is incapable of using 4g. Apple didn't get 4g until the iphone 5 it's predecessors are only capable of 3g.
Even if your on a 4g network your phone can only use 3g.