To solve this problem you must use Permutation:
You want to made 4 digit numbers with the digits 1, 2, 3, 4, 5, 6 and no digit can be used more than once. So, we have:
(A,B,C,D)
A can variate in six (6) differents forms.
B can variate in (6-1)=5 differents forms.
C can variate 4 digits.
D can variate 3 digits.
Keeping this on mind, we have:
=6!/2!
=6*5*4*3
=360
The answer is: 360
