CITROËN C4 Handbook page 260

In the event of a breakdown
Changing rear lamp bulbs
(type 1)
After removing the lamp unit, carry out the
following operations to replace the failed bulb:
F remove the two bulb holder fixing screws,
F spread the four tabs and remove the bulb
holder,
F turn the bulb a quarter of a turn and change it.
258
Changing rear lamp bulbs
(type 2)
After removing the lamp unit, carry out the
following operations to replace the failed bulb:
F remove the three bulb holder fixing screws,
F spread the three tabs and remove the bulb
holder,
F turn the bulb a quarter of a turn and change it.
To refit, carry out these operations in reverse
order.
Take care to engage the lamp unit in its
guides, while keeping it in line with the
vehicle.
Tighten sufficiently to ensure sealing,
but without forcing, so as to avoid
damaging the lamp.