If you just want to count the 4 digit combinations possible using 0-9
then you can do it this way.
You can choose the first digit from 0-9, so there
are 10 choices. Once you have chosen the first digit, you can choose
the the second
digit from 0-9, and thus there are 10 ways to choose the second digit
for each choice of the first digit. Hence there are 10 10 = 100
ways to form a two digit combination.
Once you have a two digit combination you can choose the third digit
from 0-9, so for each 2 digit combination there are 10 choices for
the third digit. Thus 100 10 = 1000
possible 3 digit combinations.
If you want to list all the 4 digit combinations then
do so in an orderly way. One way is to think of each 4 digit combination
as an integer with
possibly leading zeros. That is think of 4302 as four-thousand three
hundred and two and 0013 as thirteen.Hence you can list them as follows.
The samllest is zero
and one hundred
The largest is nine-hundred and ninty-nine