9000.The first digit can be any one of 1-9 (9 choices).The second digit can be any one of 0-9 (10 choices).The third digit can be any one of 0-9 (10 choices).The fourth digit can be any one of 0-9...
Highest is 9999, lowest is 1000. These and everything between them have four digits.Try subtracting 999 from 9999. If you can´t, there´s always wikianswers.