I phone 4 start up not loading (previously jailbroken)
You won't have bricked the phone. At most, the software will be corrupt. You'll be able to fix this. Do the following:


Leave the phone to charge overnight, to try and get off the low battery symbol. Try to use a wall adapter rather than the USB port on your computer, as the wall will provide more power.
Connect the phone to your computer.
Enter DFU mode: Hold the power button for three seconds. Then, hold the home and power buttons for ten seconds. Then, release the power button and keep holding the home button until the device appears in iTunes. In DFU mode, the screen will be completely black.
Restore the iPhone as usual.