Question 1001359
.
There are 9 options for the first digit (1, 2, 3, 4, 5, 6, 7, 8, 9);

10 options for the second digit;

10 for the third, and 

10 for the fourth digit.


In total, there are 9*10*10*10 = 9000 options for 4-digit extension.