Is there any way that someone could find out someone's 4 digit security code to unlock a moblile phone? (Other than the obvious trial and error method)
Once in, is there any way of retrieving old text messages AFTER they have been deleted?
Basically, no. You could have the phone unlocked, but then it would be in your posession and I am assuming this is not your phone. Or it could be done by Factory reset..but to do this the phone has to be in unlocked mode to begin. Boffins can extract data from a deleted message the same way an 'erased' programme from a computer can be retrieved, but it requires the right people to do it and they are no your average guys.