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

there are 9 possible numbers: 0,2,3,4,5,6,7,8,9. These can be combined in 9^4 ( 6561) ways if numbers can be repeated (i.e. 0000 or 9999 for example) or in 9 x 8 x 7 x 6 ( 3024) ways
http://answers.yahoo.com/question/index?qid=201109...

Since all normal digits are used, there are 10000 possible combinations. Thats quite a lot of possibilities!
http://www.kgbanswers.com/howmanydifferentcombi...
