Your spectator examines eight double blank cards, four regular cards, and a postcard - everything is as it should be.
The postcard is set aside, and the spectator can even place their hand (or their drink, or anything they like) on top to keep it safe.
Meanwhile, the regular cards are all sandwiched between the blank cards, and they begin to vanish, one at a time, under everyone's nose!
When all four have melted away, leaving only the blank cards, the spectator themselves lifts their hands, turns over the postcard and reveals ALL FOUR regular cards!