This exercise is meant to be done in your head to give your working memory a workout.
Think of four four-digit numbers. Let's say the year you were born, the last four digits of your social security number, the last four digits of your telephone number and the number 1234 (but feel free to come up with your own).
First arrange them in your head in ascending order.
Next arrange them so that the third digits are in descending order.
Finally, with the numbers in the order of step two, think of the four-digit number whose first digit is the first digit of the first number, whose second digit is the second digit of the second number and so on.