CITROËN DS 4 Handbook page 254

Changing foglamp bulbs
These bulbs are replaced from outside the rear
bumper:
F pass your hand under the bumper,
F turn the bulb holder a quarter turn and
remove it,
F turn the bulb a quarter turn, pull it out and
fit a new bulb.
For reassembly, carry out these operations in
reverse order.
Take care not to touch the silencer;
there is a risk of burns if changing a
bulb within a few minutes of switching
off the ignition.