Here's some info regarding the terms 904, 998 and 999.
In the Chrysler parts catalog under group 21 automatic transmission parts only the 904 is listed.
To determine the number of disc used, you must have the PK number. In all the Mopar performance books the 998 and 999 are covered but not in group 21 in the parts catalog.
Any comments or corrections are welcome.
By the way, the 4028815 shows 5 clutch disc.