The difficulty here is the repeated letters: .

We could have “TCIS”, “ATTS”, or even “MMAA”.

So we need to count three possibilities.

1) All four letters are different: (permutation of 8 letters, 4 at a time)

2) Exactly two repeated letters. Like "ATTS"

3) Two letters each repeated. Like "MMAA"

No you try to finish.