I recently upgraded from X2 to X4.
I have the .enl file and .data folder backed up (eventhough not compressed) in external hard drive and another site.
Initially when I tried opening it, the EndNote file lost all the references and when I checked the file size, it is now 0KB and all the pdf in data files is gone.
By importing the back up .enl file and data folder, I managed to get it opened properly the second time.
But this time all the groups have disappeared, and references are now in AllReferences and Unfiled.