You may not know about this feature of iphone in iOS which actually allows you to add replace the 4 Digit passcode with a alphanumeric password which definitely provides more security for the data stored on your iphone, which involves both your personal data and apps data in which you may have your username and password stored.
You don’t need to install any app for this, this is in built in iOS 5.x.x so if you are running the same on your iphone or higher you just need to enable this feature under iPhone Settings >> General >> Passcode Lock as shown in image below.
Under passcode lock, you will first need to disable Simple Passcode to off mode as you can see in the image below, you will need toggle this off
Once you toggle the simple passcode off, you should tap Turn Passcode On and now it will allow you to add a alphanumeric password which will be next time required to unlock your iphone.
Once you set a alphanumeric password you will see this one screen keyboard to unlock your iphone as shown in the image below.
[ Thanks LifeHacker for Sharing This Tip ]