How to Save Battery Life on an iPhone 4
The iPhone 4 is the popular mobile phone manufactured by Apple that gives users many options for mobile computing, but its many features can quickly drain battery power. The battery on your device may drain quicker if measures to conserve battery power are not taken. Keeping your device out of the sun, making sure your iPhone is not connected to a computer in sleep mode, turning off the display, turning off Wi-Fi and dimming the screen brightness will help you conserve battery life.
Disconnect your iPhone from a computer that is in sleep mode, standby mode or is turned off to prevent the battery from draining.
Press the "On/Off Sleep/Wake" button located at the top of the unit in order to lock it, turn off the display and save battery. The iPhone locks if you do not touch the screen for a minute by default. Unlock your iPhone by pressing the "On/Off Sleep/Wake" button again or by pressing the Home button and dragging the slider.
Turn off the Wi-Fi. Go to "Settings," "Wi-Fi" and set the Wi-Fi to "Off." Battery life may be improved by using Wi-Fi instead of cellular data networks if you use your iPhone for Web browsing frequently.
Dim the brightness of the screen. Touch "Settings," "Brightness" and lower the default screen brightness by dragging the slider to the left.