You will need to make a discount rule that is specific to that item. in the discounts tab add a new rule, for the trigger select 'quantity of a specific item in the cart' then specify the item id of that item and specify that the quantity is '>=' to 4. This is the trigger, for the amount of this discount you will choose 'based on an items column value' and enter the items id and choose the price column.
This will give a discount equal to the price of this item if 4 or more of this item are purchased.