There are 10,000 4 digit combinations possible with the Digits 0-9
Including the leading zero, then the answer is 210. Or, allowing for repeating numbers, 10000.
If every number can be used as many times as you like, there are 104 = 10000 different combinations. If each number can only be used once, there are 9!/(9 - 4)! = 5040 combinations.
Too many to list. Allowing leading zeros, there are 5040 such numbers. 4536 without leading zeros.
10,000