Any software recommendation to erase my iPhone

Hi all,



I received an iPhone 5S for a bonus in a contest. And i am going to sell my own iPhone 4S on eBay. However, I need to delete all my personal information before selling it out. The question is that the default method (Reset->Erase All Content and Settings) seems not working on my old device. Nothing happended when I pressed the button Erase iPhone. When i reboot my device, all the settings are the same and my personal data is available from the public who gets the phone.



I am wondering is there any way to fix that problem and totally remove my data on the device? Any input will be great appreciated.