Buy X get Y

I have 1 store with 5000 products and 60 different brands.

I’d like the client to be able to add 3 products to the cart of 1 specific brand and then be able to add the 4th free of charge, meaning she can add 3 different products from this 1 brand and then go back to the shop page and go select the 4th product and when added it will cost $0.

Then if she adds a 5th it goes back to the regular price until she adds 2 more.

So, 3 products, 1 free at her choosing as long as its in the same brand. I’ve managed to built everything and integrate with the brand but can’t make the 4th free product work, it only seems to work on the same product.

Any ideas? Thank you.