Separate the four Jacks from the rest of the deck. Then, take four random cards from the deck and hide them behind the Jacks, making sure that the random cards are ON TOP of the Jacks.
Show the audience the four Jacks, but not the other four cards. Tell them that the Jacks are magic.
I have four Jacks. These are magic Jacks.
Place the cards on top of the rest of the deck. The four random cards are still on top. Tell the audience that you are randomly placing the Jacks in the deck, but actually, you place the other cards in the deck.
I will place the Jacks into the deck randomly.
Use your "magic" to coax the cards to the top of the deck. Reveal to the audience the four Jacks on top of the deck. Enjoy!