Preparation

When the spectator is not watching, place all four aces onto the top of the deck.

Method

1. Tell the spectator to cut the deck into two piles. Then, have them divide the two piles into four piles.
2. Let's call the piles "pile 1", "pile 2", "pile 3" and "pile 4"; "pile 4" being the top pile with the Aces. (Don't tell the spectator these numbers. They are just for this card trick tutorial to keep track of things.)
3. Have the spectator pick up pile one, put the pile's top three cards onto the bottom, and deal the (now) top three cards onto the other piles (2, 3, and 4. One card to each pile.)
4. The spectator continues, in order, to do the same with the other three piles. (You just point to each pile, when you want them to use it.)
5. There IS no step 5, you're done! Just turn over the top cards to reveal the Aces!