To select your files you can use a filter.
Let me give here some explanation.
In Siren the file filters can be used in different places. The most obvious is the "Filter" combobox in the main window. During a directory (tree) scanning only the files passing the filter are taken into account.
You may have noticed that for the selections, highlights ... a filter can also be used.
For example, if you want to automatically select (checkboxes checked) only the "mp3" files, press Ctrl+S (or menu option Select/Select with a filter), enter "*.mp3" (without the ") and click OK.
But that's not all. The filter schema in Siren is not only the standard DOS one. I've extended it to the unix shell one. The help contains a very small description of this. I think it is adapted to your problem.
You surely already know the DOS '?' and '*' characters symbolizing any character (?) or any string (*)
In Siren this list has been extended with: [-]!
This allows you to specify specific character sets or intervals.
For example, if you want only the mp3 files beginning with 'A' or 'B'.
The corresponding filter can be: [AB]*.mp3
For the mp3 files beginning with 'A' to 'Z' it can be: [A-Z]*.mp3
If you don't want mp3 files beginning with 'A' through 'Z', it can be: [!A-Z]*.mp3
Note that I've not invented this, it is part of the standard unix shell.
It has nothing to do with regular expressions.
Let's come back to your problem:
To select/filter ... only the files with base names ending with at least 4 digits, you can use a filter like:
Of course you can use ';' (semicolon) to concatenate different filters: *.mp3;*.txt
I hope that this text will help.