JabRef 4.0.0-beta
Linux 4.10.12-200.fc25.x86_64 amd64
Java 1.8.0_131

In the entry table, sorting by year does not work, whilst sorting by timestamp works.
The log tells me:

Unable to parse stored date for field 'timestamp' with current settings. Clear button in calender popup will 
Unable to parse stored date for field 'date' with current settings. Clear button in calender popup will not work.

Also, the timestamp is not updated anymore (whilst it was updated with earlier versions of jabref).

I wonder whether some larger issue with dates exists - given #2806