6.4. There is a 36-digit number, in which the digits $1,2, \cdots, 9$ each appear 4 times, and except for 9, all other digits are less than the digit that follows them. It is known that the first digit of the number is 9. What is the last digit of the number? Please provide all possible answers and prove that there are no other answers.
6.4. The last digit of this number is 8.

According to the problem, only 9 can follow 8. If all four 8s are located within the number, then each of them is followed by one 9, and adding the one 9 at the beginning, there are a total of 5 nines.