"While many of us know about and use the simple four digit passcode or more complex alphanumeric passcode to secure the data on our iOS devices, I recently discovered an interesting option built right into the system that I find to be more secure than a simple passcode and more convenient than an alphanumeric passcode.
As mentioned in a recent episode of the Mac Power Users podcast, you can make your passcode more complex without complicating it by using more than 4 digits without involving letters and still using the num pad.
While a simple passcode is a 4 digit number, turning off simple passcode and setting up a complex passcode using on numbers allows you to benefit from the ease of use of the lockscreen numpad while having more security than a simple 4 digit passcode.
Setting this up is almost exactly like setting up the default passcode. Simply navigate to Settings > General > Passcode Lock, then be sure to set the Simple Passcode switch to OFF. Now tap Turn Passcode On and your keyboard will appear with letters, but choose the numeric view and enter the multi numeric passcode of your choice.
You will need to verify your passcode, then depending on how soon you have your device set to require passcode entry, it should immediately be activated.
You could even opt to use more digits (or as few as a single digit for more added convenience), but personally I find 6 digits to be the secure sweet spot.
While 4 digit passcodes are plenty secure and certainly more secure than no passcode, using the complex passcode option leaves intruders clueless as to how many digits to even guess adding an extra layer of security without having to fiddle with the alphabetical keyboard."
Source: How-to: Strengthen your iOS security with a multi-numeric passcode