The 4s has a SIM card. It may need to be reseated or replaced. The 4s is an unlocked phone due to having a SIM card. On the right side of the phone there is a slot and pin hole. Power off the phone, not in sleep mode, power off. Use an unfolded paper clip to open the slot by inserting the paper clip into the hole; which opens the slot. It the SIM card fall out put it back in the way the slot is designed to hold the card. Close the slot, turn the phone on. If the SIM card required message appears, reset phone to factory settings, and contact your carrier for a replacement sim. The higher iOS is reading the SIM card which is causing the error. The 3G phones where not provisioned for 4G service. With 4G service SIM cards are required the phone is old technology with new technology capabilities.