Hi, Amber. I've implemented this in what sounds like the same scenario.
Name one of the four Inventory Categories. All items that are included in that category for pricing discounts would have one answer (perhaps INCLUDE) and for the other items leave it blank. When the process is run, it will group all items in a single category (INCLUDE) together to do it quantity calculation. Once it calculates the total quantities for INCLUDE items, it then looks at the price codes for each individual item to come up with each individual item cost.
Is this what you're looking to do? For my customer, we had an inventory group for TINS, for PLASTICS and blank. It's been over a year since I've done this, so I hope I remember correctly.