We've given 9 peoples in which 4 of them are always together and remember all of the 9 peoples are not same
Now we consider those 4 persons who are always together as 1 unit
[1st 2nd 3rd 4th] 5th 6th 7th 8th 9th
So we have 6 things or 6 people to be arranged

BUT
Inside only we know there is a group of 4 people who can also interchange their positions

Hence our final answer will be 17280