iPhone 4 Battery Leaked if I Don't Charge to 100%
This sounds like it could be an issue with the your battery's fuel guage. Lithium Ion batteries have a problem with the meter that detects how much charge is left in your device. If you don't full cycle the phone every 30 days, the meter will get worse and worse.

Full cycling involves:


Charge the phone to 100%
Do not charge your phone again until all the battery is depleted (i.e. until the phone dies on it's own).


This is recommended both by Apple:


  Be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down).


... and by Battery University:


  Batteries with fuel gauge (laptops) should be calibrated by applying a deliberate full discharge once every 30 charges. Running the pack down in the equipment does this. If ignored, the fuel gauge will become increasingly less accurate and in some cases cut off the device prematurely.