Default PDF file link action changed but this is not documented

In jabref 3.8 Filename format pattern

\bibtexkey\begin{title}_\format[RemoveBrackets,RemoveLatexCommands,Replace("\s,_")]{\title}\end{title} 

and File directory pattern

\EntryType

used to work.

In JabRef 4 seemed to switch to using [bibtexkey], [title] and [entrytype]. However, I could not find any documentation of this change.

Also, it’s not clear how to apply formatting to fields, like title above.

Yes, since JabRef only the bracket syntax is supported, as documented in the changelog. The syntax is described at https://help.jabref.org/en/BibtexKeyPatterns but the documentation not yet complete. If you have a bit of spare time we would really welcome if you could work on https://github.com/JabRef/help.jabref.org/issues/179 (just click on edit here: https://github.com/JabRef/help.jabref.org/blob/gh-pages/en/BibtexKeyPatterns.md).