Although the old iPhone 4 or iPhone 4S can be updated from iOS 7 to iOS 8 easily, there are actually many iOS 8 problems you may face if you want to update old iPhone to the new iOS. So many iPhone fans choose to resell their outdated iPhone 4/4S after buying the newest iPhone 6 and iPhone 6 Plus.
If you want to sell your outdated iPhone 4/4S, be sure to clean (erase) your iPhone data to reduce the risk your sensitive personal data falling into the buyer. Because some of the data such as notes, messages, personal photos and videos may be very important personal info for you.
Remember that no method, except destroy your phone and storage, is 100 percent safe to prevent your personal data from being stolen by others. This means that data can still be recovered with the right tools. (There are many iPhone Data Recover Tools for people to recover messages, contacts, notes etc.) For example, people can easily retrieve deleted text messages on iPhone even without iCloud and iTunes backup by using PhoneRescue.
Apple devices that support iOS 5 or higher include hardware encryption when you set a passcode.
If you perform a cleaning method listed below, the encryption key is superimposed, making it difficult for someone to recover your data.
1, The first step before starting the reconfiguration process be sure to disable all services, starting with Find My Phone (find my phone). You can do this in Settings> iCloud> Find My iPhone. Enter your Apple ID access code.
2, The second step is to log out iCloud. Go to Settings> iCloud and look Sign Out at the bottom of the page. For iOS devices 7, select Delete Account (Delete account). If you delete all your data manually without leaving your iCloud account, this will also delete all content on iCloud.
3, The third step involves disabling and log out of all services including iMessage and Apple ID. To iMessage, go to Settings> Messages> iMessage. For your Apple ID, go to Settings> iTunes & App Store> give a hint to your email in Apple ID and then Sign Out. If you want to be calmer, will want to log individually from other services and applications such as Facebook and Twitter.
4, The fourth part is to start the cleaning process. Go to Settings> General> Reset> Erase All Content and Settings (Erase all content and settings). Confirm your selection.
Related Reading: How to Erase Privacy Info from an iPhone
If you registered your Apple device via the serial number, remove your profile support to enter your Apple ID in http://supportprofile.apple.com.