Trade in

See: http://www.apple.com/iphone/trade-up/

Your cellular carrier may have other offers.

Note that the prices listed on the link Foxfifth provided are the most optimal you may get for a device, not any promised or guaranteed amount. Your specific device's actual trade in value will be determined on site at the time of trade in and will depend on the actual model, configuration and condition (see the fine print for more info).

Note on the trade in page where you can estimate your own device's value, that basically anything but pristine fully functional condition on a 4s reduces it value to $0.

With a device that old, you'll almost certainly get more by selling it yourself. Just be sure to prepare it properly if doing so, as explained at What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

Hi

Not a lot you would get more money on Ebay sell it your self .

Cheers

Brian