I have the same issue, on a CDMA iPhone 4s, the battery was starting to go out and I replaced it today, this is now a backup phone but when it was my primary, I remember it being a very hot phone, even when doing something as simple as texting or calling. After I replaced the battery I was doing some testing to make sure I did everything right and it was really hot, so I took the back off again and touched the battery, nothing, touched mother board about 1 inch below the camera, and it almost burned my finger. What confuses me is nothing could have shorted before the battery replace, before that the phone had never been opened, it was more hot after the replace, but it was very hot before also. I don't know what to do. Thanks for your time reading this and hopefully I can get this fixed.