When you factory reset an iPhone 4, you're returning the iPhone to the state it was in when it left the factory: no music or other data on it and all the original settings. You're completely erasing the phone and starting over from scratch.
Circumstances in which you'd want to factory reset an iPhone 4 include when you're selling it, when you're sending your iPhone in for repairs, or when there's a problem with the iPhone so severe that this is the only way to solve it.
Here's how to do it.
- Begin the process, if possible, by backing up the data on your iPhone 4. Do this by syncing the phone (each time you sync, it creates a backup). This lets you save all your data and settings to restore to the phone after the reset.
- When that's done, locate the Settings app on your iPhone homescreen. If you've rearranged your apps, locate it wherever you put it. Tap the icon.
- Scroll down to the General menu. Tap it.
- Scroll all the way to the bottom to the Reset menu. Tap it.
- On this screen, you'll be presented with a number of reset options, ranging from just resetting all the iPhone 4's settings to resetting its dictionary or homescreen layout. The factory reset option is Erase All Content and Settings. If you're sure you want to factory reset your iPhone 4, tap it.
- You'll see a warning reminding you that you'll be erasing all music, other media, data, and settings if you do this. If that's not what you want to do, tap Cancel. Otherwise, tap Erase iPhone.
- It will generally take a minute or two to factory reset the iPhone, but when the process is done, you'll have a brand-new, pristine iPhone ready for whatever your next step is.