In base ten, there are 9000 four digit number, the numbers are from 1000 to 9999. If you allow leading zeros, then there are 10000 numbers. [0000 - 9999]
12 numbers
2240
-4
2
If you want 4-digit numbers, there are 24 of them.
24
17
3 digit numbers are lessthan 4 digit numbers
There are 2941 4-digit numbers such no two of its digits differ by 1.
There are 16 4-digit numbers that can be made up from 9 and 6.
-2
12 numbers
There are 90 such numbers.
There are 9000 4-digit numbers in hindu-arabic numeration system
9999
4
90