It's not "adjustable" as such... the height is regulated but there's only one mode for driving. All Citroën's with hydraulic suspension have hydraulics on all 4 wheels, they just tend to sink ass first when you leave them standing for half an hour.
The only car I know off with Citroën hydraulics at the rear only is some special high performance version of the Peugeot 405 (and maybe the last Traction Avants that served as prototypes for the system).
The only one I've ever driven was brand new when I was in Spain as a kid.
It had a lever which made the rear deflate or sit very high.... It didn't do anything to the front!
It was broken then.
