It looks like you're asking what is the largest 4-digit number which has four 4's for digits - with a four digit number using four 4's, there is only one such number: 4444. Maybe I'm not understanding the wording of your question.
You get the largest number if you sort the digits, from largest to smallest.
Using the digits 12279, the largest number that can be represented is 97,221.
7654321
543,210. If you can use them all multiple times, then it is 555,555.
97432
99
If the digits are only used once, each, the largest possible number would be 97,531
The largest even number that can be made with those digits is 9740 .
The largest odd number that can be made with those digits is 9407 .
985,316
543210
the largest five digit number that can be formed using the digits 7 and 9 is 97,000