Try clicking the question mark next to the 'Version Number Format String' text field, the behavior is well documented.
> The version number format string is used to generate the version number. It will be passed through unaltered, except that any expression of the form ${variable_name, argument} will be replaced according to the table below.
The format string '${BUILDS_ALL_TIME, XXXX}' works beautifully.
0001
0002
0003
etc.
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