Unlimited
103 = 1000 in all.
900 ,999 ,909
10 x 9 x 8 = 720 different "permutations"
If the first digit can be zero, there are 5,040. If the first digit can't be zero, there are only 4,536.
9
1000
There are 210 4 digit combinations and 5040 different 4 digit codes.
10,000
103 = 1000 in all.
900 ,999 ,909
10 x 9 x 8 = 720 different "permutations"
104 or 10000
If the first digit can be zero, there are 5,040. If the first digit can't be zero, there are only 4,536.
9
There are 10000 such codes. Each of the numbers 0-9 can be in the first position. With each such first digit, each of the numbers 0-9 can be in the second position. With each such pair of the first two digits, each of the numbers 0-9 can be in the third position. etc.
1,000 of them. The list of possibilities will look exactly like the counting numbers from 000 to 999 .
The possible 4 digit codes using the numbers 0-9 are every number between 0 and 9999. For numbers that have less than 4 digits, just precede the number with 0's. 10,000 possibilities