Keep your iPhone Stuff Locked.
That four-digit passcode of days past is no longer the standard. Apple pretty much insists on six digits to keep your private content, well, private. And hey, that's a good thing.
But if you've got some seriously sensitive stuff, you can always go alphanumeric. A combo of letters and numbers gives you a virtually unlimited number of possible passwords, meaning hackers (or mistrustful significant others) probably can't break in.