Battery Life Issues
1) why does my phone not hold it's charge longer anymore?
2) why does my battery life show 20% then drops instantly to 8%?
3) why does my phone shut off automatically when it's between 4-6%?
How old is it? Battery life is finite - you only get so many discharge/charge cycles before performance will degrade to the point of seriously affecting performance. But the battery began degrading the minute it was assembled and continues to do so all along.
If you suspect a premature failure, make an appointment at an apple store and they will check it out for free.