There are 3 choices for each of the 4 digits, so the total number of possibilities is
3 x 3 x 3 x 3 = 81 different 4-digit numbers.
99999
10
15 of them.
5040 numbers can be made.
24 of them.
There are 60480 numbers.
99999
There are 4^6 = 4096 such numbers.
8.
125
10
13579
15 of them.
5040 numbers can be made.
there are 22
6, if you don't repeat any numbers.
24 of them.