Got a used iPhone 4, how to reset?

Okay, well my uncles friend had an old iPhone 4 he didn't need anymore, so he gave it to my uncle, who gave it to me. But, I'm trying to reset it, but the original owners iCloud data and iTunes account and stuff is on there. So I can't turn off iCloud and reset it so now i'm kinda just stuck with some other guys phone. (Which doesn't have a sim card in it, mind that). So instead of waiting to go to the AT&T store, is there anything else I can do in the meantime to just wipe the whole thing?
Usually I don't ask online when it comes to this stuff, but I'm just not sure right now. So, any help would be greatly appreciated, thanks in advance! <3

Added (1). Also, no, we can't contact the original owner because my uncle moved, and does not have any type of connection with him anymore, whatsoever.

Well if its asking for a Apple id and Passcode there's nothing you can do even if you have a sim. The only way to get pass it is if the original owner sign into his iCloud online and delete the account from that phone or he type it in person. So there's really nothing you can do at this point unless you get ahold of him. You can still use it just that you can't sign in to your apple id and download app.

Setting->general>reset