I had a similar problem with my 4s. It would be on the charger for hours and get extremely hot, not hold a charge etc.. I took it back to my service provider (Verizon) complained about the situation, left the phone with them for several hours and... Same as you they couldn't replicate the problem! I told them I wanted my phone replaced, I was still under 1yr manufacture warranty like you. I was pretty irritated at the whole ordeal at the store, and the manager understood my aggravation and took care of me and I had a new phone within a couple days. I also have 5 lines with them and a large data package so they may have taken that into consideration as well.