Please select a product from the buy 1 get 3 free category only. Later again select 3 products from the same category. Now after you select total 4 products and proceed to the payment page automatically you will only have to pay for 1 product from the 4 of the selected products. You will have to pay for the one which is of a higher price the other 3 products which are of a lower price becomes free, you don't have to pay for them.