The mentalist shows a black bag and states that it contains the 4 kings. It asks a viewer to think of one and name it. He then takes the cards out of the bag, fan them out and the king he thinks will be the only one turned face up among the others. After a moment of suspense the mentalist turns the chosen king back up and everyone can see that it is the only one with a different colored back.
Easy to do. Perfect to always carry with you. The cards, laminated, are poker size.