If you have the second part “EFGH” in some property “{{mySecondPart}}” you can add a DataSet “Load a Set Of Filenames” with a file pattern “*_{{mySecondPart}}.xml” to your “Read a File Step” and use the name of the dataset as your file name.
This is assuming your dataset returns only one file that fits the dataset filepattern. If there is a possibility that more files are returned then you will have to create a loop before the “Read a File Step” and process all the filenames returned by the dataset until you hit the one you need, then from your loop-step (“Executable Script Step”?) your next action would be the “Read a File Step” .
Cheers,
Danny