how do i delete photos from iphone 4 photo library

Photo library contains photos that were synced to your phone from your computer using iTunes. They have to be deleted by syncing. Connect your phone to your computer, open iTunes, click on the name of your phone in iTunes, go to the Photos tab of your iTunes sync settings, deselect any folder containing the photos you no longer want on your phone (uncheck Sync Photos to remove them all), then click Apply at the bottom to sync your phone.