I seem not to be able to get midi to connect in Ardour 4.0. When I run Jack, ardour does not show up as a client, as Ardour 3.5 did (presumably because 4.0 is designed to function without Jack? Aj2midi is running). And the Routing grid within Ardour does not show my USB-interface any more, even though the audio/midi-setup-screen allowed me to select my interface.
I guess I just don’t understand enough about how all these connections work. Any idea where I could read up on it?
You’re pretty confused here. Ardour 4.0 can function with or without JACK. Whether it uses JACK or not is up to you. If you do not select JACK as the audio/midi backend, then naturally Ardour will not show up in JACK. And if you do not select JACK but JACK is running, then Ardour cannot use the same device as JACK.
Either use JACK as before (making sure you select it from the audio/midi setup dialog, or make sure you set up the ALSA audio/midi backend (inside Ardour) correctly (which may involve making sure that JACK is not running so that Ardour can use the correct device).