CITROËN DS 4 Handbook page 66

Back-up control
System allowing the doors to be locked and
unlocked manually in the event of a malfunction
of the central locking system or battery failure.
Locking the driver's door
F Insert the key in the door lock, then turn it
to the rear.
You can also apply the procedure described for
the passenger doors.
Unlocking the driver's door
F Insert the key in the door lock, then turn it
to the front.
Locking the front and rear
passenger doors
F Open the doors.
F On the rear doors, check that the child lock
is not on (see the corresponding section).
F Remove the black cap, located on the edge
of the door, using the key.
F Insert the key in the socket without forcing
it, then without turning it, move the latch
sideways towards the inside of the door.
F Remove the key and refit the cap.
F Close the doors and check that the vehicle
has locked correctly from the outside.
Unlocking the front and rear
passenger doors
F Pull the interior door opening control.