Jabref-4.1 is still not as stable as Jabref-3.8 was. Jabref stores some metadata such as groups in comments of the Bibtex database.
Jabref-4.x changed the file format unfortunately. I am not sure, if it was just the format how groups are saved.
How to convert a .bib database on a Linux system from the Jabref-4.x format back to the old format used in the final version 3.8.2?
Is there any feature which will get lost, because it was not known in the old file format?
Update: Thank you for the helpful comments so far. I upvoted several already. I will review now within the next days, perhaps my question is invalid or needs improvement.