Arrange cards with values ace through 9 in a row, in counting order, with the ace on the left.
Take up a card from one end of the row — left or right, your choice.
Do this twice more, each time taking up either the leftmost or the rightmost card in the remaining row.
When you have three cards, add their values, divide the total by six, and call the result n. Count the cards that remain on the table from left to right.
The card in the nth position will be the 4.