Iphone 4 replacement or fix?

I recently traded my iphone 3 for an iphone 4 (white) from my bestfriend. He had gotten an iphone 5 and stopped using the 4. As soon as i got a sim card in it, i started ddropping service. It will have no service or will be searching for service while somebody with the same phone stands right next to me and has full bars. He never had that problem with this phone and i know this because i was always with him. I was wondering if anybody knew how to fix this? I already tried soft resetting it, resetting everything to factory settings, and i got a new sim card for it but nothing worked.

Or i was wondering if I took it to apple if they woulld just replace it? I do not have warranty on it. My friend took her iphone 4 to the apple store because her lock button broke, and they gave her a whole new iphone.

Not sure