This demo project uses the gviz query language to retrieve data from a google sheet. There are no dependent drop downs on the google sheet, this is all handled by the app and the gviz queries. One assumes that the app user will not need to create stock items, so this is not included in the app. The spreadsheet owner can categorise their stock items from category lists on the spreadsheet. Only stock items will be used to generate the dependent drop downs on the app. To select an item or items using all four categories in my demo will therefore require 5 calls to the google sheet. (This is far too many for my liking, but given the desire to use a google sheet for this work....)
Stock Sheet
Category Listings
Blocks
All Stock Items
Category Selected Item
Working Demo App
ShopSelections (2).aia (25.6 KB)
open for questions 
Credits @KevinKun for the Tableviewer extension, used to display the stock items