Can I trade my iPhone in for a new one?

I want to buy the iPhone 5s. Will my service provider buy my used 4s back? Or do they let me have it as a "souvenir"?

Hum, no idea if you don't share exactly who your mobile provider is. You can call your mobile phone carriers customer service then ask, right.

Ask your provider if they will buy it.

You can't trade it, you will have to pay money for the new one.