I can only make assumptions, here...
- AFAICS, the two first ones are probably - and when I say 'probably', it's because I don't see any other context in which they could be usable - linked to one of the two #40891 or #40892 actions (
Actions window, section
MIDI editor).
- The third one doesn't appear on the
official images list for the toolbars. As it's here, I guess that it could have a use somewhere, but which one...
- The last one is used when the MIDI sources are .mid files (
Preferences>
Media>
MIDI pane). It is linked to the #1013 action (also in the
MIDI editor section of the
Actions window). It's the only one I am sure of, actually.
Sorry not being able to help more...