How do you remove the parental controls restriction or reset that (4-digit) password?

Ask your parents if they will remove it. If you don't know the passcode, then you would have to restore the device to remove it and set it up as new, not from a backup. You cannot use a backup since it contains the passcode. You will not be able to use a backup, so your phone will be set up as new, wiping all of your content.