Same here, double checked my settings. All was still attached to meet the extra bagspace requirements. Issue on Blizzard’s side i assume, for those who wonder what happened to items in those 4 unfortunate slots =check other bags for they just have moved items to free normal slots OR if bags are full, they have been sendt into a mail back to you. Some players have been having issues getting the extra bag slots from adding an authenticator to their account and Blizzard have explained its related to out of date addons: Bad Slots Fix (source) I cannot get mine to pop, Blizz account even says the authenticator is attached.

After the release of patch 7.3.5, players who attach a Blizzard Authenticator and SMS Protect to their Blizzard Account will be eligible for 4 extra backpack slots. It can take up to 30 minutes for the backpack slots to be applied to your World of Warcraft account after these security features are added and may require logging out and logging. Bagnon ReagentBankButton is a simple Bagnon plugin adding an extra menu button to toggle the reagent bank. I like being able to toggle the reagent bank on and off, which is normally only possible with the reagent bank button in Bagnon's 'Bag Slots List'. Any player with an authenticator already attached to their WoW account will see the additional 4 slots on their backpack when they log into version 1.13.5. It can take up to 30 minutes after both security features have been added before your extra backpack slots are available —log out.

How to Get Additional 4 Bag Slots

When you add the Blizzard Authenticator and Blizzard Phone Notifications to your account, your primary backpack size is increased. If you remove either the Authenticator or Phone Number, your backpack reverts to its original size and the items in those slots are moved to free slots in another bag. Any player with an authenticator already attached to their WoW account will see the additional 4 slots on their backpack when they log into version 1.13.5.

It can take up to 30 minutes after both security features have been added before your extra backpack slots are available —log out of the game and back in if they don’t unlock. If you wait 30 minutes and still can’t use the extra slots after relogging, reset your user interface to resolve any possible conflicts with addons.

Note: If you have no free bag space when the extra slots are removed, the items will be added when you free up bag space, or they will be mailed to you. World of Warcraft Classic characters do not receive additional bag slots.

The Bag slots or Inventory^[1] are located above the Micro menu which contains a backpack and 4 slots for bag items. The default backpack is a 16-slot container, and it can thus hold sixteen items – one in each slot. New bags you acquire can be smaller containers, or bigger than your backpack.

Some people considered the key ring button part of this area, but it has since been removed.

Your inventory in the game. All items not equipped on your character are stored here. One backpack is automatically provided for you. To fill the other four slots, you must purchase, find, or make additional bags.^[2]

While the bar has five slots only four are changeable, the slot at the right-most end contains the backpack, which cannot be removed or replaced. Attempting to remove a bag from its slot with items will result in the message 'Can't put non-empty bags in other bags.'

Wow 4 Extra Backpack Slots Not Working

Bank slots

Main article: Bank

Each character can purchase up to seven bag slots in the bank. The cost of the bag slots in the bank increases with each extra slot purchased, starting at 10, then increasing to 1, then 10, then 25 for the rest, for a total of 111 10 for all seven slots.

Empty bag slots that do not have a bag equipped cannot hold items (other than bags).

Wow Classic Extra Backpack Slots

Gallery

• Bag slots prior to patch 8.0.1

• Bag slots before the removal of the keyring

• Backpack icon

Wow Extra Backpack Slots Not Working

Patch changes

• Patch 8.0.1 (2018-07-17): Moved off the Interface Bar to the right corner of the screen along with the Micro Menu.
  
• Patch 4.2.0 (2011-06-28): Removed the keyring.
  

References

1. ^Blizzard Entertainment. 'Chapter 5', World of Warcraft: Game Manual, 65.
2. ^Blizzard Entertainment. 'Chapter 3', World of Warcraft: Game Manual, 18.