Used Iphone 4 used for Wi-Fi only?

I'm thinking about buying an iphone 4, but i plan on using my wi-fi instead of getting a plan. Will I be able to do this? I'm going to be buying the Iphone used. Please don't ask why not just buy an ipod, the person who is selling it to me is giving it at a good price. & also, the seller had at&t. If decide to activate the iphone will i have to activate it wity at&t?

Yeah you can just use it for Wi-Fi, like and ipod but to activate it to another service you have to unlock it. My brother uses an iphone 5 for Wi-Fi and music etc