My iphone 4 will shut off and not turn on. I tried holding the power and home button at the same time and this seemed to help for a few minutes. When this happens, the phone will be fully charged. I have trouble turning it back on. Also, when it gets to about 30% battery life, it will turn off. I also make sure to close out apps when I am finished with them because I know this will kill the battery. What can I do to fix this? Please help!