You may have to bring it in, or call a T-Mobile store specifically. I was quoted $300 for my factory unlocked and mint iPhone 5, which I will be selling elsewhere for more of course. A factory unlocked iPhone 4 in good condition can easily go for $200 on eBay or Craigslist. T-Mobile won't pay anywhere near that for yours. You're better off selling on your own in order to get more and compensate for the newer phone.