buy one get one free

We sell items in singles but there could be between 2pc and 24pc to a pack. We would like a bogof offer that if one of the promotional items was put in the basket (4pc in this item) it wouldn't split the pack and dive it into 2 and 2 free. We would like it that you put in the 4pc and only get the buy one get one when another set of 4 is added, and the cheapest item if free.