Modifiers - Promotional Goods

I am trying to set up a modifier that will provide one free item when four distinct/unique products are purchased regardless of item amount or item quantity.  The free item is always the same.  I have used promotional goods modifiers in the past for similar scenarios but cannot figure out how to use it for this scenario.   I am also trying to program a LIMIT ONE PER CUSTOMER SHIP TO address but seemeingly Limits are not available for Promotional Goods modifiers.

Many thanks,

Hector