CITROËN C4 Picasso Owner's Handbook Manual page 66

Access
Deadlocking renders the interior door
controls inoperative.
It also deactivates the manual central
control button.
Therefore, never leave anyone inside
the vehicle when it is deadlocked.
66
By the doors:
With the electronic key in the defined
zone A, press a finger or thumb on one of
the front door handles (at the markings) or
the locking button located on the tailgate to
lock the vehicle.
Within five seconds, press the door handle
or the locking button located on the tailgate
again to deadlock the vehicle.