10,000
Too many to list. Allowing leading zeros, there are 5040 such numbers. 4536 without leading zeros.
There are 10,000 4 digit combinations possible with the Digits 0-9
If you can repeat the numbers within the combination there are 10,000 different combinations. If you cannot repeat the numbers within the combination, there are 5040 different combinations.
That's exactly all the numbers you need to count from 000,000,000 to 999,999,999. There are one billion of them.