Insert leading zeros

I have a string of digits that looks like something like these: 0025, 1234, 0001, 1003, and so on. They are all 4 digits in length. Then when I increment one of them by 1, I want to still have 4 digits for the value (it should insert the leading zeros if needed). Is there any type of function that will force the incremented value to be exactly 4 digits no matter what number it increments? Please let me know if there is a function that will insert these leading zeros or truncate it to 4 digits if necessary. Thanks. Matt