With 4D 6.5's new password system, it is actually possible to "lock" yourself out of the database. This can be done by creating a user and setting up this user to be the default user in the Database Properties dialog. If you then uncheck the button "Display User List in Password Dialog Box and restart the database, 4D will display a dialog box with the User Name grayed out.
If you would like to access the database as a different user (ie: designer), simply reopen the database while holding down the "shift" key. Once you do this, you can then enter the designer or administrator name and password.