Question: how many combinations with 4 fixed numbers?
Answer: Hence, in this case there are 4 3 = 12 possible two digit numbers. For the third digit there are only 2 choices and for the last digit, one choice. Thus if you are not allowed to repeat a digit the number of possible 4 digit numbers you can construct from 1,2,3,4 is 4 3 2 1 = 24.