I'm selling my iPhone 6 to a friend who has an iPhone 4 (or iPhone 4c?). We are both on AT&T. She asked that I help her set it up. I know to back hers up in iTunes then use that backup to transfer her data to the iPhone 6. Regarding phone device, would I be able to just switch out the SIM card from her iPhone 4 to the iPhone 6? Are the SIM cards the same size? Or are we going to have to call ATT&T? Thanks in advance for your help!