Interesting discussion and very useful information. I use Fedora Linux, a git version of darktable and the fedora dt v4 package. I switched to the Adobe lcp directory an started the command
lensfun-convert-lcp --db-path /home/grimble/.local/share/lensfun/
but I was unsure where the lensfun database is located. So I ran the command with the db-path
/home/grimble/.local/share/lensfun/updates/version_1 and
/home/grimble/.local/share/lensfun/updates/version_2 and because I was still unsure
/usr/share/lensfun and
/var/lib/lensfun-updates
To my surprise the new lenses do not appear in the lens correction module. Do I missing something?