Hi, I tested a recent beta version of Jabref on Linux to manage my .bib files. There seems to be an error with the focus of new windows sometimes. If someone is interested in testing if this bug is also on other Linux systems, or even on other OSX/Windows systems, I would invite him/her to leave a note in the bugticket: https://github.com/JabRef/jabref/issues/2928
In short: doi requests work again in Jabref 4, the GUI is rewritten and still has plenty small glitches.