My 4 LTE has been having battery issues lately as well. I use to wear it 23.5 hours per day and only charge 30 minutes, I could do this all week and then sleep without the device on one of the weekend days to bring it back up to 100%. Now I have to charge it back to full or near full every morning or else it will die in the middle of the night.
This device does not have the always-on display so it will be interesting to see how that affects battery life. Google says Apple rates the 4 at 18 hours, but it says the same for 5 and 6, and I was getting +/-23 out of my 4 for 2.5 years. Perhaps I am not a heavy user of the device.