It is recommended that you don't use ios 15 system, because the configuration of 4S can't meet the requirements of 15 system, the processor is relatively backward, and the memory is very small, so it can only be used as the function of answering calls at ordinary times.

If you have to brush the phone forcibly, it is likely to cause the phone to crash, resulting in the phone being turned off or directly stuck. If you want to replace the new system, buying a new mobile phone is the best choice!