I just got my daughter a 4s, and I don't plan on replacing mine for quite a while. But I've got a drawerful of older dumb phones. Time for a run to the local Cell Phones for Soldiers drop off location.
If you are an ATT customer and you have a phone that was paid off in full through the contract.. you can have ATT unlock the phone so it can then be used on and GSM carrier. This bumped up the price on the iP4 32GB I just sold to $350.00 ... a 3 year old phone.
This may apply to other carriers, I'm sure.
All you need is the IMEI number and an active account, or be a previous account holder.
I would keep one old one as backup. If you were to lose your current phone, damage it, it stopped working or whatever, my theory is that you could activate the old one temporarily to at least make calls until you were able to resolve the problem. Maybe it would take a few days for repair or whatever.
In your case if you don't have a need for the 4s as an iPod internet device, sell it, and keep the 3G as a backup.
Like NG4 said, a carrier unlocked phone sells for a lot more, and your 4s is multi-band capable, Verizon, ATT, whoever. The Verizon iPhone 5 allows both GSM and CDMA so you can choose your carrier, but the ATT iPhone 5 is GSM only. The iPhone 4s is both GSM and CDMA. That makes it worth more. If you search the completed listing the 4s in top notch condition it's going for as much as $3-500.
For the record and off course, the 5s and 5c are multi band/multi channel too, but they are a serious notch up in technology doing GSM, EDGE, LET, UMTS and a bunch of bands I never knew existed. Seems like it covers all the extra bands, frequencies, and channles now.
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