How do I make an Item Variation subtract specific product amount from main item inventory?

[sfj2112]

Hi, i'm setting up a variation on a specific item, for customers to have an option buy a case of that item. 

I have a total of 100 pieces.

A case is four pieces.

Is there a way to automatically reduce my current overall item inventory by a specific amount (four) if a case is purchased?

Basically i'm looking for shared inventory on an item, with a multiple value reduction of inventory (x4 units) when the case variation is purchased. 

Thanks, Steve

[Goldneye · Accepted solution]

HI, unfortunately inventory stock is not able to deduct from the main stock.  It can only track individual stock inventory.