The battery of iPhone 4 is 1420mAH. If you use a lots of apps and functions inside the iPhone 4, the battery will drawn out quicker than expected. Either you will need a backup power supply to supplement the iPhone 4 usage or use some sort of fast charging method via PC to make your iPhone 4 works all the time.
There are various methods,
For fast charging
For as backup battery
Check to see if which one works for you at http://thechinacellphone.com/online_order_iPhone_iPad.htm