Basics of setting up Inventory:
The Base Unit is what you sell.
The Transaction Unit is how you buy it.
Additional Units are used for alternate measurements of the Inventory Item, and you can buy by these Units as well.
My preference is to leave Transaction Unit blank/empty, and instead define an Additional Unit as the Default Transaction Unit (check the box next to the Unit line).
Your Consumption will be reported by Transaction Unit if you do not leave it blank. So if it is defined as Case with a Multiplier of 12, when you sell 1 Can, your consumption will report 1 of 12 = 0.833 of a Case Consumed, with Instock level reported as 0.916 of a Case left. This is not useful in scenarios such as this.
If you leave Transaction Unit blank, your Consumption will be reported by the Base Unit. Most of the time Base Unit Consumption makes more sense. So when you buy 1 Case (of 12), and sell 1 Can, you will see Consumption reported as 1 Can, and your InStock level will be reported as 11 Cans left.
Besides Inventory Items, you need to define Recipes for all your Items, as such:
P.S. The screenshots are from V5, so you can ignore fields that are missing in V4. The concept is still the same.