iPhone 4s getting hot
Hot while using the phone is almost always a short. You might have it serviced if you think it's a fire risk.

Hot while charging can be a bad charger or a bad battery or a short, so you've got the easier problem to troubleshoot / repair.

Also - the age of these devices mean that the majority will be failing in one way or another whether the batteries need to be replaced or other issues like corrosion or just failure of another component has caused it to be out of design spec.