I'm willing to try your port to my J4+. What should I expect?

Same problem here (as well as at https://forums.ubports.com/topic/262/desktop-apps/69 and at https://github.com/ubports/ubuntu-touch/issues/22, for reference).

Cheers

Thanks for the update.
I just noticed that on an already installed libertine you need to edit
~/.cache/libertine-container/first/rootfs/etc/apt/sources.list
changing the URLs therein to
http://old-releases.ubuntu.com/ubuntu

Cheers