Once your client has gained access to all their photos using the 4-digit master PIN, they get to see all their albums that have already been published by you.
By navigating to the 3 vertical dots on the side of the album tile, they will get the Lock to Guest option that will make that particular album private. Therefore, your client's guests will not be able to view that album once they log in using the 3-digit guest PIN.
If your clients have already opened an album they wish to lock, they can navigate to the 3 vertical dots on the top-right corner of the screen and use the same Lock to Guest option to make the album invisible to their guests.
Further, if they want to lock multiple albums in one go, your clients can navigate to the menu bar on the top right and enter Share Settings.
Find all the albums that are unlocked or locked and simply press on the lock/unlock button to make the album private/public in just one click.
There is an additional option to share even the locked albums using individual album PINs that keep the privacy of your clients intact, letting them share the locked albums with PINs to only a selected few guests.