Sure the four digit passcode on the iPhone is cool, but looking for something a little more secure you can enable an alphanumeric password on your iPhone with a free utility Apple provides.
Then launch the utility, go to “Configuration Profiles”, “New”. Give it a name and identifier, then go down to Passcode, enable it and set your own options.
Once you’re done, go to your phone in the left pane, “Configuration Profiles” tab, and click install next your profile. It’ll pop up on your phone and you can install it!
Simple and secure way to lock down your iPhone a little more.