Ok, need a variation, namely:

1. customer buys 4 of product A and then gets 1 of product X free
2. customer buys 8 of product A and then gets 2 of product X free
3. customer buys 12 of product A and then gets 3 of product X free

So in any case the quantity ratio A/X is 4/1

Can Better Together do this without much fuzz?

There will only be one single combination, namely A + X, no such thing as B + X or B + Y etc