The 4S, 16 gig, black and in good condition gets you $200 at Verizon. The deal is you have to buy a new 6, 16 gig minimum and must trade in the old device. You can get an accurate quote on the Verizon site. The deal was till 0/30 but as stated, was extended till 10/15.
You pay $200 up front for the 6, cannot roll the purchase to an existing bill, then get a $200 gift card for the trade-in, which you can use on your account.
The usual $30 upgrade fee applies, but it's still a good deal if you are OK with a 2 year contract.