Hello,
On my Zynthian V4 with last Stable I don’t have any activity on the midi jacks or led for IN and OUT.
Plus, if I use a midi contrôler or a midi-usb adaptater I don’t have midi out neither.
I don’t see any menu where I can control that,
Is it working great on your side ?
Check out this (brief) description in the user documentation. You have to enable MIDI output, per device, per chain.
Oh, I didn’t see “output” on screen … But pretty sure the ports are checked.
I will try again this evening
Thank 
By default, all input ports are enabled and all output ports are disabled. This meets most users’ initial preferences and relies on each user to tweak this to better suit their configuration and use case.
Oh !
I did reburn an uSD last month !! That explain why it worked before and not now !
Sometimes there us stuff that you apply one time and forget…
Many thanks for the information! 
I had a little time available so… Bad new :
There is no Midi Out per chain
There is Midi Out per device in the admin menu BUT there is no check box for DIN-5 MIDI. So I guess this one should be always active.
So… It seem I have an hardware problem with my Zynthian V4 or Midi câbles.