If you allow zero to be in the first positions (0000 and 0001, etc) then there are 10,000 combinations (0000 to 9999) If zero can not be in te first position, remove 0000 to 0999
http://wiki.answers.com/Q/How_many_possible_combin...

You have 5 choices for the first number, 5 choices for the second,
http://www.quora.com/Mathematics/Howmanycombinat...

You have 10 choices for the first digit, then 10 choices
http://www.chacha.com/question/whatareallofthe...

4 digits means less than 5 digits and greater than 3 digits. This eliminates all numbers greater than 9999 and all numbers less than 1000. How many numbers are there between 1000
http://answers.yahoo.com/question/index?qid=200907...
