Hi @technase 
This is Ellie from BOGOS, #1 Promotion App on Shopify.
May I ask did you use Shopify Discounts or apps to create the Buy 4 get 1 Free?
If you use Shopify Automatic Discounts, you’ll need to add both products X and Y to the cart, the discount will then make product Y free.
On the other hand, BOGOS offers a hassle-free solution for you. When customers buy 4 products, the gifts can be automatically added, simplifying their checkout experience.
Feel free to explore the demo of this offer here: https://bogos-demo-store.myshopify.com/collections/free-gift-auto-add-to-cart (password: 1234)
Grab 7-day free trial of BOGOS (4.9 overall rating & 2,045+ reviews) to give it a try! 
Wish you all the best with the business,
Ellie
BOGOS (formally Free Gifts by Secomapp)