I looked in the manual and searched the forum, but I cannot find a way to control the internal sequencer of A4 (start/stop or just change the pattern) by MIDI or automation. Is it possible?
If it’s not possible I will have to check how to use the “SongPos” mode in the OB…
Sequencer start stop is just a MIDI clock/transport thing - perfectly possible.
As for pattern change you can do this by sending MIDI notes to the hardware via OB…or at least you used to be able to. Something tells me that might have got broken in the last OB update. It wasn’t something I ever tried.
I should’ve started with the OB manual:
“PATTERN CHANGE
You can send MIDI notes to the plugin on channel 16 to
_ request pattern changes on the Elektron device. Like _
song position sync, this works in a best effort manner.” 
Will try that today.
As I say, I have a feeling that stopped working in the latest update.
Thanks for the heads up - will try that and will post my findings 
EDIT: Had a minute and checked that - it’s working in OB 1.15.0.
With Multi Map you can also change patterns with different midi notes ranges (Direct Jump, Direct Start, transposition…)
One more question - is there a way to control FILL button (“Page” to be exact) via MIDI?
Nope
shame. So FILL trig condition can only be used by pushing the button on the unit - nothing else?