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You have four digits to place: __ __ __ __
In the first position, you have 4 choices of numbers.
Once the first digit is placed, you have 3 choices left for the second position.
Once the second digit is placed, you have 2 choices left for the third position.
Once the third digit is placed, you have only 1 choice left for the last position.
This gives you 4x3x2x1 = 24 possibilities.