I think you want to know how to combine multiple items sold and not yet paid for. On a desktop computer you can go to Orders awaiting shipment and next to 1 of the items click Send invoice. This will generate a combined invoice of all 4 sold items. On this invoice go to Discount and enter the shipping discount. Then send the invoice. If the buyer uses this invoice to pay, you need to do nothing as the original (4) listings are what make up this single order.
There are ways to set up shipping rules in advance of sales for multiple sales of a single item or combined sales of different items that will automatically include shipping discounts. Let me know if you'd like info on how to do this as it's not obvious and it can cause errors if done incompletely or incorrectly.
