Once again, consider the permutations when selecting three letters from {A, B, C, D}.
A,B,C
A,C,B
B,A,C
B,C,A
C,A,B
C,B,A
A,B,D
A,D,B
B,A,D
B,D,A
D,A,B
D,B,A
A,C,D
A,D,C
C,A,D
C,D,A
D,A,C
D,C,A
B,C,D
B,D,C
C,B,D
C,D,B
D,B,C
D,C,B
Notice how these 24 permutations fall into only four distinct categories if the order does not matter. Therefore, from a group of size four you can pick an unordered subset of size three in only 4 ways rather than the original 24.