Because the order of the groups doesn't matter, you divide by the number of permutations of the four groups, which is 4! If you don't, you treat each group uniquely, and one group will be group 'A', one 'B' and so on, so the arrangement A,B,C,D is counted as different from B,A,C,D or D,B,A,C, or any other permutation of the 4, even though the groups are the same, leading to an overcount of the number of combinations.