On sprint can you upgrade iPhone 3G to iPhone 4s?

After 2 years do you have to upgrade to the regular iPhone 4 or can you upgrade from the iPhone 3G to the newest iPhone 4

Yes, but got to 4S at least as thats free on a 2 year contract.

You realize that they stopped making the iPhone 4 like 2 years ago? The 6 is about to be released, current is the 5s and 5c. You might even get a 5, but why would you want a 4s?

Buy a phone on eBay or Craigslist and just extend without signing a contract. IPhone 5 or 5s, When the iPhone 6 comes out in September, sell the phone and use that money to get the 6. Invest and you shall be rewarded. Btw: carriers will unlock the phone and this sells for a lot more. Do this before you get the 6.

If you really, really, and I mean really want to a $1 4s on a new two year contract, you'd better hurry. Why? The 6 will be out in <90 days and when it hits, the $1 4s is gone forever from all of them. You don't have to. You could get a 5c for $99 if you want. But after a 3g it's time to upgrade. I still have my old 3GS in the drawer. See the link, it might surprise you. Best luck.
http://shop.sprint.com/...HE:iPhones