Buy 3 Different Items of Same Size Get 1 Free

Good Day

I run a Shop that sells prepared meals, we have fresh and frozen dishes, we have different sizes(300g, 700g etc.) And we have different kinds of dishes(beef, chicken, vegetables, etc.)

I want to create a special for the frozen items,(buy 3 dishes of one size and get 1 dish of that same size for free) but the variety of different items are massive, so the key factor of the free items has to depend on the size of the items and not the names

And I want to be able to have a button to turn this feature on and off as I sell fresh and frozen items and only want to use this feature on the frozen items(fresh and frozen have the same name, barcode, id, etc.)

I am asking a lot from you experts, but I need help to do this

There is an easy way to just manually mark off the free item but employees don’t always concentrate on things like that and might miss an item and that results in unhappy customers.
Thank You In Advance😉

So how do you differentiate between different sizes? Tag? Portion? etc.