Simple answer, a combined invoice is not possible, I do not think ebay intends making it possible, as they make a bit more money on each transaction.
As already said, only way to combine is for seller to cancel and relist as one item.
Other way is you pay for each one and seller then refunds excess postage, BUT posting all items as one, leaves seller only with evidence of one delivery and 4 with no proof of delivery and you could open Item Not Received Cases.