4 digit or 6-digit PIN makes no sense on OS X, but is that actually on iOS 7, and not OS X ?
Tell us which Mac you have, please …
And, can you explain exactly when that PIN is required?
Is that a user login (which also shows your account name), or is it a simple blank line, with a padlock on one end?
Finally, why do you believe you are limited to only 4 characters?